Protohippus is an extinct three-toed genus of horse. It was roughly the size of a modern donkey. Fossil evidence suggests that it lived during the Late Miocene and Early Pliocene, from about 14 Ma to 6 Ma. Analysis of Protohippus' skull and teeth suggests that it is most closely related to the genus Calippus. Species include: \n* P. vetus \n* P. perditus \n* P. supremus (also P. simus) \n* P. gidleyi