A Morgûl-Horse
By the end of the Third Age, the Dark Lord had succeeded in reshaping some stolen foals from Rohan. 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.
Notes[]
By its description the mount of the Mouth could have been also some sort of machine or automaton, or maybe a hybrid half horse half mechanical construct.