arion

 In Greek legend, a fabulous horse, the offspring of Poseidon by Demeter (or, in other accounts, Gaea or a harpy) who to escape him had metamorphosed herself into a mare. It was successively owned by Copreus, Oncus, Heracles, and Adrastus. It possessed marvelous powers of speech, and its right feet were those of a man.  [Century Dict. 1906]