Nightmares, sometimes called Demon Horses or Hell 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. Nightmares had pitch-black coats with huge head, sharp fangs, and dark eyes often illuminated by the red-hot flames that emanated from their mane, hooves, and tail. They smelled of brimstone and possessed unnatural agility, as well as the ability to fly.
Nightmares would only allow those of sufficient wickedness and power to ride them. When free, nightmares stalked those they could victimize, running through the Ethereal Border Plane before suddenly rematerializing and attacking their target. Rather than simply murdering their victims, however, they did everything they could to torment and strike terror. After ensuring fear in their target, they waited, allowing hope or safety to be felt before delivering the killing blow with tooth and hoof. They tended to hunt in small, predatory herds similar to Wolf packs.