Nightmare

Nightmares are a form of Devil native to the Astral Domain of Hell. They resemble black horses with flaming hooves and manes, but they are actually fiendish creatures as intelligent as the average Human. Although they are mostly used as mounts and the like by other devils, sometimes a powerful devil might give a mortal agent on Tol the favor of riding one.
Like most devils, if they are slain on Tol, they return to Hell in a flash of brimstone. If they are actually slain in Hell, they are killed for good.