The Collaborative International Dictionary of English v.0.48:

Arion \Arion\, prop. n.
   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]