Monster Lore: The Pegasus and Their Nightmare

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 Purity Of The Pegasus

  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...

The Twisted Amalgamate Of A Nightmare

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.

The Story Between The Two

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