The monsters of Dungeons and Dragons sometimes fall into the swirling cyclone of the diametric.  Two sides of an arcane coin, unequivocally linked to one another.  Some grow out of opposition, while others are twisted transformations of one - a dark miasma of spitting fire and screaming madness.  
  With all my talk of pendulums swinging and balances shifting, my lens lingers on creatures of this influence.  They are fascinating to consider - beyond our stats and alignments, to explore the STORY of their nature and what gives way in the imagination's gates when you do.

  The white winged horses known as Pegasi soar through moon-touched skies, a vision of grace and majesty.  When glimpsed by mortals, they linger only for a moment, touching down for a drink from mountain springs or crystal lakes.  Intelligent and vigilant, any sound or sign from the local wildlife will send them back into the safety of the clouds above.

Born in dens of starlight in the realms of Arborea, the overwhelming positive good of a newborn Pegasus would spill out into the astral sky, alighting new stars and sending the nearby plants to bloom.  An equine marvel hatched from an egg, a Pegasus embraces flight almost immediately, its feathered wings glinting with astral efflorescence.  
  Pegasi nests are commonplace in Arborea, and the wondrous sentience of the creatures makes them loyal mounts to the Seldarine - the pantheon of Elven gods.  Faster and calmer than any wyvern, a Pegasus must be persuaded to serve their rider, entering a partnership that can last millennia.  The Seldarine will sometimes send Pegasi to the Material Realms to aid their followers or serve as messengers of their will, but depending on the forces in play, sending these faithful creatures has its own risks...

A visage of hellish terror, a Nightmare often appears in a cloud of roiling smoke, its mane, tail, and hooves wreathed in fire.  The creature's onyx form moves with a wraith's speed, vanishing into the night in a burst of brimstone.  
  Though it can be ridden by the occasional antihero on a redemption kick, the Nightmare is a prime mount for creatures of exceptional evil.  Demons, devils, death knights, and liches all call this fiendish horse a steed, and many more repugnant souls seek to claim their own by summoning one from the Lower Planes.  Thing is, if you don't feed that hell horse a worthy sacrifice upon its arrival, it has no reason to stay loyal to you.  A good many cultists have been burned to a crisp due to this small omission in the fine print.

This is where things get interesting.
  According to Dungeons & Dragons canon, a Nightmare isn't BORN, they are MADE.  It takes a dark ritual involving the torturous removal of a Pegasus's wings, the more brutal the better.  Literature is mixed, but the theme here is cruelty; the slower and more horrific the removal, the greater the Nightmare produced.  And this isn't a bait and switch situation; the Nightmare IS that tortured Pegasus, driven insane by brutality and dark magic. 
t's sick.  And I kind of love it.
  Because this relationship raises a few pathways that I want to consider:
First, the inner circle of the Seldarine would be indirectly responsible for the creation of a Nightmare in most cases, as these dark rituals often take place at the hands of mortals - or those directly tied to them.  To send a Pegasus as an avatar of the Seldarine opens them to possible capture through dark forces, perpetuating a dangerous silent war of astral attrition.  How insane to fathom an Archfey riding into battle only to come face-to-face with their old trusty mount, twisted by darkness and brutality.  
Second, the Nightmare retains the intelligence of Pegasus form.  This is no mindless beast bent to one's will, and though the rituals require sacrifice to link the mare to the rider, I would venture that a mental or verbal bargain is still required.  In fact, I could see the maddening steps of a certain ritual play out deliciously - fool a Pegasus into believing it is making a heroic pact and companion, only for the creature's true nature to slowly twist the poor being into its Nightmare; like a frog slowly boiling in the pot.  
  You're not breaking down a pet, you're convincing a sentient creature - this requires more nuance.  I like to think that every Pegasus has a name, a personality; hopes, dreams, goals, not unlike a heroic PC.  How does one twist and manipulate a hero into becoming a villain?  If the Star Wars prequels taught us anything, it was how NOT to portray one's turn to the dark side (sudden is bad writing) - what if this process could be more insidious?  Slow and manipulative, with a grand payoff.
Finally, the monster lore stated that the more brutal the shift, the STRONGER the Nightmare.  Which pulls me down the jackal hole.  If you have a particularly twisted or prolonged "ritual", could you create a "greater" Nightmare?  I'd imagine that Pegasi under the Seldarine would make names for themselves similar to their heroic riders.  Perhaps the greater the renown, the harder the fall.  Such a change might yield something along the lines of a Night Terror (or "Knight Terror"); the twisted mirror of a once great heroic steed.
  And if this is possible, could a Nightmare be redeemed?  Perhaps its flight is never restored, but its good nature and starlit white coat, with blue fire for its crest, restored.  What story could that creature tell?

As I research, I leave you with one more idea through the ether.  A Nightmare canonically can pass through the Ethereal Plane, carrying its rider and several others along for the journey.  Imagine what it sees in the mist of the dead.  Old soldiers failing to pass on?  The spirits of lingering beasts and the wisps of old sages?  Or do they see themselves...the Pegasus inside, a haunting specter of their past life. 
And does a Pegasus dream of its other possible lives?  Does its innate senses of fate and danger paint the picture of its dark future, its Nightmare, in cool reflections of mountain springs.  

The pendulum swings.  
Take care, lovely people.
-Adamus

A cursed mix of wings, talons, and a vampiric stag's head, the Peryton is a monstrous entity.  It is depraved relentless in its vicious pursuit to maim and devour the hearts of other creatures, holding a specific hatred for humans and elves.  Even injured, these creatures will hunt down detected foes, until at least one of them is dead.

Their feathering from a distance could be considered beautiful, and with wings tucked, settled upon the ground, the Peryton might pass for a lesser woodland being.  The moment one gets closer, though, you'll notice a good many things are off.

First, actually, would be the smell.  In this case, a lack of one.  Less sensitive noses would identify the creature as human, but those with a modicum of training or enhanced primal senses would pick no smell at all, coupled with an overwhelming sense of dread.  
  Standing over 7 feet tall, the Peryton's demonic stag face smiles with rows of razor-sharp teeth.  The plumage of their chest might tell you their gender (males tend to be blue, while females are a pale white), but soon you won't care.  Their antlers, jet black and harder than steel, are used to rend and impale, and their depraved form is resilient to all but magical weapons. 
As their eyes of orange flame pierce through the fog, any light that passes over them lies about their true form.
  For you see, the most peculiar element of a Peryton is their shadow.  This winged, evil beast will hiss before you, but its shadow will present as human.  Always.  Human.  

This oddity presents a plethora of curious theories, but few had evidence to match.
  Some Loremasters believe the false shadow to be an echo of the last creature the Peryton killed.  Given its penchant for violence against humans and elves, this was the prevailing theory for hundreds of years.  It wasn't until the haphazard findings of one Grenaldi Mayweather, a gnomish priest under the cover of twilight, who one night happened upon a nest of the creatures in the Ghastshadow Mountains.  She observed the flock tear into a pack of roaming Aarakocran, ambushed or set upon as they slept.  Just as history told us, the Peryton ripped open the chests of their victims, consuming the still-warm hearts with grim satisfaction.  Mayweather watched in dread curiosity, eager to solve the riddle of the shadow...  But the humanoid echo remained.  No winged shadows manifested.
  Mayweather was lucky to escape with her life, but returned soon after for more observation, this time with an elite guard to watch her back; the knowledge was too important not to be careful...

It would also be Mayweather that would witness the first instance of a Peryton being born.  Though there are documented family units of a male and a female Peryton (though they'd be hard-pressed to care for their young, if not kill them), this instance within the nest involved two females.  
  Both left to hunt, smelling blood on the wind.  They were gone for hours.  When they returned, blood dripping from their claws, they delivered several chunks of flesh to the next to feast upon, and a still writhing human man!  Mayweather watched as her lookout - a young brown-eyed ranger - was clutched by one of the pair and pulled toward the other, like it was presenting a gift.  Firey smoke and twisting gray tendrils spilled from the one in waiting, before its teeth sank into the man's chest, ripping his beating heart from the cavity.  His body went limp, and Peryton female scarfed down the heart, her eyes shifting translucence in the penumbra.  
  Then Mayweather heard a sound that sent chills down her spine.  
Like an echo of a child's laugh.  A giggle in the gloom, small and innocent.  Then it grew, warping and dancing along the walls, raising in pitch and warble, until it is a chorus of cackling.  The other eyes in the nest rise and join in, their necks convulsing and twisting backward with the sound, a malevolent inversion of mirth and satisfaction.  The sound becomes a beast in and of itself, a roiling mass of whoops and hollers, striking chords and stark dissonance.  Vibrations spill out the mouth of the cave, whispers at the ears of the hidden spies, and shouts upon the walls that surround them.
  The guards around the gnome begin to clutch and claw at their ears, the cacophony bringing a few to tears.  Mayweather instinctually covers the sob of her closest ranger, insisting that he get himself together.
Then.
Silence.
  The smell of dread hits Mayweather's nostrils as she dares to peer into the nest.  All the Peryton stare straight ahead, their mouths gaping open - a frozen, terrifying smile.  Until they all snap their eyes...to her.

They made it out with one casualty.  Her second blue-eyed scout, snatched by a rogue talon before he could slip into the wooden door of a magnificent mansion.  
  Safe within the dimensional space, Mayweather vigorously wrote down her conclusions:
1. New Peryton are born by a female consuming the heart of a freshly killed humanoid.
2. Their shadows flicker into monstrous forms during the process of incubation.
3. When incubation begins, nearby Peryton gain heightened senses to protect the pregnant female, becoming even more violent.  The more there are, the stronger the pack's senses.
  Her fourth note she added weeks later, when her guards - while out hunting - were set upon by a pair of Peryton, seemingly tracking them for miles.  When they finally confronted the creatures, they were surprised to see two young Peryton, newborns - one with brown eyes and the other with blue, both with shadows of human form.  ...Felling the creatures was tough, but much worse for their psychology.  Poor guys.
4. Newborn Peryton share the eyes of the creature whose heart they were born from.  As they get older, and kill more for their own survival and pleasure, the blood tarnishes the eye color, shifting it to match the orange-red of the others.

Mayweather and others theorized that the first Peryton was a cursed human or elf, twisted by a god of chaos.  The bards of old expanded upon this, marking its origin to infidelity, curses, and carrions feasting on cursed corpses.
  Given Mayweather's most recent expeditions, however, the clearest line exists through pure vehemence.  These aren't cursed humans - they were MADE by something.  Sometime in the first age, when great magics could be wielded by mere mortals, a wizard general - whose name is lost to time - sought a tactical edge against the elven and human alliances.  So he juxtaposed what he had on hand with fiendish blood.  An intellect unmatched, the alchemist rivaled the gods for a moment - before his heart was ripped from his chest...

Mind the shadows.  Watch the skies.
See you next time.
-Adamus

These little buddies are so intrinsically in tune with the magical world that I have taken the liberty to codify a few with magical persuasions of their own.  Sure, you'll have the standard pseudodragon flapping around your nooks and crannies and old libraries, but if you dig deep enough and scour long enough, you'll undoubtedly run into one of these variants.
  Illusory - Pseudodragons don't have a specific language, instead communicating telepathically simple ideas.  Those that are steeped in illusion magic has found a way to interpret the innate basics of their school to help communicate better, manifesting in a sort of "thought bubble" above their heads communicating more intricate picturesque ideas.  I just love the idea of a pseudodragon with an ellipsis over its head as it ponders, and then watching it blip back and forth between steak and its studies; the ultimate SQUIRREL moment.  :)
  Evocata - a little friend who spends a lot of time around battle mages and explosives can evolve into a combustion dragon.  Highly emotional, passionate, and excitable, an Evocata will manifest its core magic in explosive bouts.  Get too excited?  Fireball.  Or Fog Cloud.  Or Plant Growth.  Get too sad?  Cast Darkness.  Get protective of their master, their binky, or their new bestest friend that they've known for five minutes?  Ray Of Frost.  Or Finger Of Death.  You know, TOTALLY NORMAL reactions.
  The list continues, and I have a feeling the more I play with this concept, the more hilarious it will become.  Does a Cleric use her Pseudodragon as her own personal lie detector because it's infused with Divination magic?  Does a diplomat use his to win over a crowd because he steeps in Enchantment?  The possibilities abound!  
(and now my players know...good luck)

See you at the table.
​-Adamus
  

Pulling our visions into messy arcane studies, pits flanked by dangerous ooze, and dark caverns of mystery, we rest our eye upon one of my favorite little critters in this whole game.
  Sporting leathery skin and angry spikes, with claws for climbing, and a mouth often gaped, the Nothic is unnerving on its own, but its enormous, unblinking eye sets it far apart from its other cave kin.  Despite its stark appearance, though, the Nothic can form rich bonds and motivations, even spilling themselves into an industry all their own.

Like I said, the Nothic uses to be a spell caster - a creature devoted to the arcane.  This means that those cognitively aware enough of their own affliction would seek to remedy it.  Which births Nothic alchemists and artificers, brewing elixirs and constructing shadows, seeking to silence the incessant voices of their fragmented memories.  
  For those who drink the ill-made elixirs, further mutations plague their forms.  Orcish blood infuses their body with resistance; devilish magic turns their blood to smoke, their steps to shadow; Fey magic infuses the spines and spurs with wild magic that explodes with their emotions.
  On the other side, the unhinged artifice breathes new terrifying life into their visage.  An extra set of arms of offset their twitchy nature; electric infusers to inject them with strange mutagens, all the better to defend their workshop from intruders.  If you were driven to mad industry to silence the voices in your head, what would you make?

See you at the table.
-Adamus

In 5th Edition Dungeons & Dragons, you'll likely run across the Neogi in three forms - the Hatchling, the standard Neogi and the Master.  Let's break them down.

HATCHLING (tiny) - young and fledgling (AC 10-11; HP Level 1 Wizard), these are dangerous in large numbers and simple on their own.  Just don't wake up their older siblings.
NEOGI (small) - better natural armor (AC 15-16) and multi-attack makes this bugger a tougher beast to tango with, but this standard beastie comes with the main thing that makes these critters tick.  The Neogi has an Enslave ability (Short or Long Rest dependent) that pushes a Wisdom save.  Fail that, and you're charmed by the creature for one DAY (and it knocks out your Reactions - bummer).  Oh and did I mention that the Neogi talks to you telepathically during this whole thing out to a distance of one mile?  *shudder*
MASTER (medium) - not to be confused with the "Great Old" variety, these nasty variations on a theme get everything their small counterparts do (plus more than double their HP pool), and are freaking spellcasters!  Looks like these masters went all in for the Hadar fan club with a slice of Confused Bard, so be ready for some telepathic Vicious Mockery while being whalloped with Eldritch Blast for good measure.  Be careful too, they can function as a 7th-9th level caster, so that spell save may be meaner than you think at low level.  And I know I mentioned the "Hadar vibe", but any DM that does their research can mix up that power set to match another Elder Evil (wink-wink).

Now, each of these creatures has the Mental Fortitude of an Elf and Spider Climb, so watch your six.  What they lack in empathy, they make up for in numbers, and if they're following that Spartan Chuck-E-Cheese childhood, you can bet that what's coming for you ain't no pushover.

The Neogi are a Legacy Monster.
  Given their expansive and extensive lore dating back to second edition (2e), not to mention their pronounced influence within the Spelljammer history, their current iteration and inclusion feels lackluster.  There are dozens of fascinating creatures with deep history and ethos in Faerun and worlds beyond that are offered only a tiny passage in Volo's Guide, or Mordenkainen's Tome, or the Monster Manual talking about them, and, unfortunately, many of these passages end with a "nobody really knows" and a shrug (flips page).  
  To say that these are missed opportunities isn't exactly fair, either.  At its core, 5th Edition is intended to be accessible.  You don't want to bury the DM in mountains of back-logged history (some of it quite problematic today) just to get an interesting critter on the board.  So instead, I offer up my own approach:
This awesome monster can be a one-off.  A "well, that was weird, moving on" type moment.  OR, much like what happens when I do my research for each of these, this monster is a launching point of inspiration.  Truthfully, I hadn't yet considered the Neogi's place in my homebrew setting, but you can BET I've got one mixing now.  The rabbit hole I jumped into has some really cool avenues to play with, and I can twist and turn and mutate them however I see fit.
These things have a huge spider motif going for them - could they be distantly connected to Lolth?  Maybe the Drow that serve her hate Neogi, because they believe their banners as an affront to their queen!  You don't know (shrugs and cackles in DM).  
  My point is that for every moment of disappointment found in a feature, creature, or spell, there can instead be a tiny mote of inspiration, leading you to a setting with greater depth, immersion, and personal craft.

Happy worldbuilding.
​-Adamus

Though the origins of the creature rarely differ, their intentions in Io are often less miserable.  They are also HEAVILY influenced by the plane that they exist in.  
  WoWs of the Feywild will be more innocent and mischievous - still deadly, mind you - but their souls of dead sprites and nymphs don't understand the weight of their actions.  It's tragic, really.  These playful spirits are drawing creatures into their sight to feel not so alone...which kills their new friends anyway.
  WoWs of the Shadowfell carry a sorrowful weight and an unending desire to consume the living.  These spirits are tormented by the wales of their own death that they seek rebirth by draining the life force from other beings.  This usually manifests in alternative versions of their Consume Life ability, wherein they don't kill the target.  Instead, they inhabit their body, siphoning the creature of each of its attributes until only a husk remains...and a Wight is born instead.
  The more I write on these creatures, the greater the permutations and variations that manifest across the planes.  I consider what is in the Monster Manual as fact for the Material Plane, but as any Horizon Walker knows...Planeshift is one helluva drug.

Be safe out there, and beware the lights that drift in the forest.
-Adamus

Will-O-Wisps can be found all over Dungeons & Dragons, but for 5th Edition, look no further than the classic Monster Manual.  

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Displacer Beasts were once the prized jailers and guards of the nobility that filled the upper ranks of the Unseelie Court.  After the Massacre At Harrowhome, however, their prized position grew scattered, and one too many instances of their allegiance being swayed fell dim upon the Dread Queen Mab.  In the Ruins of the Season, deep in the harrowed halls of the broken citadels of beasts and men, prides of Displacer Beasts roam and hunt.  
  But not all nobility has turned a blind eye to these creatures with a love for the kill.  Excellent trackers, a Displacer Beast may sometimes roam with hunters and assassins of a deadly order.  Creatures contracted to hunt their own, these "Skipjacks" utilize the Displacer Beast as a dangerous ally in their pursuit of their fellow Fey.

GM's Note: Since the first campaign I ran in this custom setting, there have been old tapestries, panoramas, ancient texts, and obscure references that player-characters have stumbled across.  Each lore drop would be a tiny morsel of a much larger picture; an eight-legged displacer beast with boney spurs around the jaw-line, multiple tendrils lifting from its back like a living mantle, and a crown of black and silver spines protruding from its head.  This regal creature was always set upon a high rock surrounded by hundreds of other beasts, each bowing to it as if it were their king.  
  The first time this image was presented, I remember my players vigorously writing it down.  At any point following this, any players to be present during this strange initial reveal would bring it up again if ever they fell into an encounter with a Displacer Pride, but little came of it.  
​  Will we see more?  Only my players can tell you...

Tread safely under the borealis of the Feywild.
-Adamus

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Mephits are imp-like creatures of mischief and chaos forged in the elemental planes.  They prefer to dwell in places where their base elements are prevalent, so a Mud Mephit may reside in the plane of earth or water, while a Magma Mephit adores earth and fire.  As pairings go, the Elemental Chaos has forged six slices of these little critters.

DUST - composed of earth and air, Dust Mephits are drawn to death.  Adventurers can encounter them in deep catacombs, deadly streets, or in the eye of a shredding sandstorm.

ICE - a creature born of frigid air and water, an Ice Mephit is devoid of pity.  Aloof and cold, they show little mercy or compassion to those around them.

MAGMA - dangerous to the touch, a Magma Mephit is comprised of earth and fire.  Considered the least intelligent, they are slow to understand commands and are often used as living walls of molten death by more cunning creatures.

​MUD -  slow creatures of earth and water, Mud Mephits are full of complaints and seek endless attention and treasure, stuffing the little trinkets inside their warping bodies.

SMOKE - crude and lazy, Smoke Mephits billow constantly.  Though they are often gifted with superior speech, they like to use it to spin lies and lead creatures astray.

STEAM - nearly ethereal in sight, Steam Mephits can be tracked by the trails of hot water they leave behind.  Fire and water yield bossy and self-absorbed creatures, and each will tend to appoint themselves lord of all other mephits.

Dust and Smoke find great meaning in Ionian Shadowfell, as both can be easily satiated by the natural surroundings, but they are certainly not alone.  Given a Mephit's ability to squeeze, melt, and morph, they are highly suited and trainable in acts of espionage, information, and theft.  It is this practice that birthed the Smoke and Shard - a Mephit spy network created by a vampire lady in the Court Of Whispers.
  Even the Mud Mephit's greed can be entwined into valuable service.  A Mud Mephits for treasure and reward gives it purpose in its employ, and it can promised boons and glittering prizes for tasks and services.  Though slower than the cruelest among them, a Mud Mephit, given enough time, can be trained well to serve a single master, as long as that master has kept their promises of treasure.  Given the immortal nature of a mephit, as steadfast as the element that creates them, a master with a similar lifespan and the correct resources could secure a loyal servant indefinitely.  When considering the vampire Court that rules the Ionian Shadowfell, such service would be invaluable... 

So when you see that precarious mound of magma just sitting there in your path, think twice about kicking it.  You just might upset all of its brothers.  
Tread carefully.
​-Adamus

And that does it for December, and 2020!  Up next?  Not sure.  The polls haven't closed yet.
  Celestials and Fiends are extensive.  We'll see who wins next week.  Happy New Year, friends.
​-Adamus

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In terms of raw defenses and hit points, the Yeth Hound isn't that intimidating, but make sure you're packing something silver.  Their physical stats (Strength, Dexterity, and Constitution) are noticeably high, but they're dumb as a post most of the time.  
Where they become truly dangerous is in their unnatural, creepy mobility and their dreaded "Baleful Baying" howl ability.  The thing's got a 300 foot range, so make sure your Wisdom is decent before engaging with one.  If you happen to be one of the unlucky low rollers against the howl, that frightened condition does a lot more for the Yeth than it does for you, stacking on extra psychic damage for its scared victims.  And I can't blame them; the image of this thing bearing down on you is NOT pleasant.
  As an aside, I was actually very intrigued to write about this thing.
​There is something wholly unnerving about these large Fey doggos with humanoid faces, who creepily hover overhead and paralyze prey with their baleful howl.  It's just such an unsettling image.  Add on to this the fact that as long as it's on the same plane as its master, it can ALWAYS contact it telepathically.  Woof, buddy.

Sleep well, travelers.
​-Adamus