The Horse Evolution faq from Talk Origins discusses the evolution of horses: At this point in the early Eocene, equids were not yet very different from the other perissodactyl groups; the Hyracotherium genus includes some species closely related to (or even ancestral to) rhinos and tapirs, as well as species that are distinctly equine. [Note: the particular species that probably gave rise to the rest of the equids, H. vassacciense, may be renamed, perhaps to “Protorohippus”.]