Hipparion \Hip*pa"ri*on\, n. [NL., fr. Gr. ? a pony, dim. of ? a
   horse.] (Paleon.)
   An extinct genus of Tertiary mammals allied to the horse, but
   three-toed, having on each foot a small lateral hoof on each
   side of the main central one. It is believed to be one of the
   ancestral genera of the Horse family.