Nightmare (creature)

Nightmares, sometimes called hell horses or demon horses,  were fiendish equines named after the terrible dreams suffered by those that encountered them. They were wicked creatures best known for serving as mounts under a variety of malevolent beings throughout the Lower planes.

Description
Nightmares were seemingly emaciated stallions, approximately tall at the shoulders, with night black coats. On further examination however, their warhorse-like exterior was revealed to be entirely superficial. They had huge heads, fangs like vipers, malevolent dark eyes often illuminated by red hot flames and spouted orange fires when their nostrils flared. Wreathed in fire, their manes were wild and their tails unkempt, but turned to cinders and quickly dispersed upon death.

Their steely hooves left scorch marks wherever they went but the tracks left mysterious patterns and gaps. They galloped through the night with unnatural agility, their black bodies blending into the darkness leaving onlookers with only the sight of racing, infernal flames that appeared and disappeared in smokescreens of brimstone. Rumors had it that the creatures moved so fast that they could only be seen as a blur, but those that had viewed them speculated that they could move between shadows or teleport, which would explain their ability to stop and start in an instant and suddenly appear without seeming to have moved.

Personality
Nightmares were callous and cruel creatures that stalked reality searching for those it could victimize. They displayed hatred towards all life of the Material Plane and exhibited an infernal intelligence unseen in true beasts. Rather than simply murder their targets, they used their supernatural powers to strike terror in them first, making them fear for their lives and killing them once they were reduced to panicking wrecks. One of their favorite tricks was to allow their victims one last moment of feeling safe before suddenly reappearing, dashing their last remaining hope and then striking the killing blow.

Despite being wild and roaming creatures they were known to serve willingly as mounts during missions of malice, but only for those of sufficient wickedness and ability. Those that wished to ride them would be wise to remember that nightmares were not simple steeds but fiercely independent and untamable fiends with their own instincts and agendas. Prideful and egotistical, nightmares often acted upon their own wills even if such actions ran counter to their goals of their supposed masters and sometimes even at their expense. Even if both parties desired to commit evil deeds their exact methodologies could be incompatible and attempts to force them into servitude often ended poorly. Only after forging a bond with them over time would a nightmare become loyal to a rider and only then would the two become true partners in villainy.

As Mounts
Nightmares were often conjured into the world to serve as steeds for evil priests and wizards. Sometimes they were sent to a mortal as a "reward" for service to a powerful fiendish lord. A nightmare would sometimes serve a powerful evil creature as a mount. It possessed the ability to travel to other planes and a nightmare could bring its rider to one as well. A nightmare extended its resistance to fire to its rider. According to legend, particularly powerful nightmares could also carry their riders to and from the Shadowfell. There, they served as steeds to powerful evil beings such as hags and demon princes. One should be careful to think a nightmare to be a stupid beast; they were as intelligent as a human, but also wicked and depraved.

Creation
Nightmares did not appear naturally. They were the result of a dark magic ritual applied to pegasi. The ritual involved the removal of a pegasus's wings by torture, followed by its transformation and abandonment of its noble qualities in favor of evil.