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: P. vetus P. perditus P. supremus (also P. simus) P. gidleyi↑ ↑ ↑