Nightmare

Picture a huge, muscular horse with a pitch-black coat, red-glowing eyes, nostrils that flicker orange flame and hooves that smolder like coals, its mane and tail ragged and wild. Nightmares understand orders from evil riders and speak to one another by empathy.

Their hatred of life drives them to assault any creature not native to the lower planes, and sometimes natives as well. A nightmare bites for 2d4 and lashes out with two burning front hooves for 1d6+4 each, igniting anything flammable they strike.

The heat of battle makes it exhale a choking, scalding cloud that fills 10 feet around it. Those inside must save vs. paralyzation or suffer -2 to attack and damage rolls while blinded and gagging.

Wingless, it still races through the sky by magic at great speed, and it can cross into the Ethereal or Astral plane whenever it pleases.

Night hags, baatezu, tanar'ri and mighty undead lords all ride them. These are clever beings, not livestock. They happily join any evil errand, yet often follow their own wishes over their 'master's', much like a magic weapon with a strong ego.

An outsider visiting the lower planes can win one, but only through a ritual by a single mage or specialist wizard of 5th level or more. First comes mount, to draw a nightmare's notice; then monster summoning III, which binds it; then wall of fog, out of which the steed charges with blazing eyes. The final step is a meal of oat-like platinum flakes worth 200 gp, and whoever feeds it commands it for the next 72 hours. Fiends never bother with this process; no mortal knows what pact they make instead.

Once a year the fiends of the gray wasteland of despair hold a grim council to plot the coming year, and nightmares herald it. For the two weeks before, the steeds become uncommon rather than very rare, and just before the council opens they sweep across the planes in a dreadful stampede.

They are not related to true horses and need neither air nor food. Though classed as carnivores because they eat slain enemies, they likely draw strength from serving evil rather than from meat.

A nightmare dying of age is said to seek a hill of bleached bones in the gray wasteland, the only white spot in that dull land, and great stallions arrange honor guards to haul their corpses there from other planes. One wizard who claimed to have seen it reported eerie quiet after the endless shrieks of the wasteland's fiendish hordes, and skulls that still snorted and neighed. He watched fog gather at the base, and an ancient, swaybacked stallion with silvered hair trotted out. It neighed once, every skull roared back, and it collapsed and died.