Nightmare (creature)

A nightmare was a neutral evil outsider from the Lower Planes. They were a type of fiend that existed in the dimensions beyond the natural world.

Description
Nightmares were evil creatures that appeared to be large black horses with flames wreathing their hooves, flowing forth in the shape of their mane and tail, and trailing from their nostrils and red eyes. If killed, the nightmare's flames guttered out, leaving a mane and tail of ash that quickly dispersed.

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.