Horse
The horse adapted to survive in areas of wide-open terrain with sparse vegetation, surviving in an ecosystem where other large grazing animals, especially ruminants, could not. Horses and other equids are odd-toed ungulates of the order Perissodactyla, a group of mammals that was dominant during the Tertiary period. In the past, this order contained 14 families, but only three—Equidae (the horse and related species), Tapiridae (the tapir), and Rhinocerotidae (the rhinoceroses)—have survived to the present day.
Caviinae
Caviinae is a subfamily uniting all living members of the family Caviidae with the exception of the maras, capybaras, and "Kerodon". The subfamily traditionally contained the guinea pig or cavy-like forms along with the cursorially adapted (running) "Kerodon". Molecular results suggest the Caviinae as so defined would be paraphyletic and "Kerodon" is more closely related to maras and capybaras than to other caviines. This led Woods and Kilpatrick (2005) to unite "Kerodon" and capybaras into the subfamily Hydrochoerinae within the Caviidae. These studies also suggest "Microcavia" and "Cavia" are more closely related to one another than either is to "Galea".