A nightmare is a fantasy monster.
They are evil horses who serve as mounts for powerful undead, minor demons, and all sorts of other evil extraworldy creatures. They have eyes that glow red, and flames often emit from their nostrils and hoofs. They have a wild, ragged, and powerful look about them (this would give away their true nature, even if the flames were not present).
The nightmares speak no language, but they can understand any evil creature that mounts them (through the power of telepathy). A wild nightmare will attack anything it encounters (even ones under the control of another creature will often attack any non-familiar creatures on sight). In combat the nightmare is a fearsome foe, attacking with flaming hoofs, and biting with their powerful jaw. Once a nightmare becomes excited from the heat of the battle he will begin to emit a cloud of hot vapor, which has an effect similar to cs gas (choking, blinding, and eventually killing anyone around).
Nightmares also have the ability to fly (even though they have no wings), and travel in several dimensions.
A person of great power and evil can summon a nightmare for service. But this is much like summoning a demon, as the nightmare will always have its own agenda, and will have no problem betraying his rider to meet his own goals.
The nightmare does not need to eat, sleep, drink, or breathe. But they will often do these things anyway, when there is no greater evil to attend to. (In other words, they often eat, sleep and such, but they suffer no ill effect from not doing any of these things.) The nightmare is not a natural creature, therefore they live forever, or until slain.
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