The forces of Evil, most notably the corrupted Maia Sauron, have bent horses to their nefarious purposes, just as they have other forms of life. Sauron originally had his Nazgûl mounted on black horses stolen as foals and thus accustomed to evil creatures and surroundings at an early age. These beasts were also fortified somewhat by spells and potions to make them swift, tireless, and unafraid of things that would terrify a normal steed. Sauron was unable to breed a strain of horses shaped to his will.
By the end of the Third Age, however, the Dark Lord had succeeded in reshaping some of the stolen foals. These unfortunate creatures were enlarged and strengthened, given carnivorous foreteeth, fiery, night-sighted eyes, and steaming, smoking breath. Their faces became skull-like, and their hooves were like polished iron. One of these twisted creatures was the mount of the Mouth of Sauron, Lieutenant of the Tower of Barad-dûr.
References
- Retrieved from [Realms of Arda]; original reference: [ICE MERP 2012]