Horse
Besides the horse, there are six other species of genus "Equus" in the Equidae family. These are the ass or donkey, "Equus asinus"; the mountain zebra, "Equus zebra"; plains zebra, "Equus quagga"; Grévy's zebra, "Equus grevyi"; the kiang, "Equus kiang"; and the onager, "Equus hemionus".
Capybara
The capybara and the lesser capybara belong to the subfamily Hydrochoerinae along with the rock cavies. The living capybaras and their extinct relatives were previously classified in their own family Hydrochoeridae. Since 2002, molecular phylogenetic studies have recognized a close relationship between "Hydrochoerus" and "Kerodon", the rock cavies, supporting placement of both genera in a subfamily of Caviidae. Paleontological classifications have yet to incorporate this new taxonomy and continue to use Hydrochoeridae for all capybaras, while using Hydrochoerinae for the living genus and its closest fossil relatives, such as "Neochoerus". The taxonomy of fossil hydrochoerines is also in a state of flux. In recent years, the diversity of fossil hydrochoerines has been substantially reduced. This is largely due to the recognition that capybara molar teeth show strong variation in shape over the life of an individual. In one instance, material once referred to four genera and seven species on the basis of differences in molar shape is now thought to represent differently aged individuals of a single species, "Cardiatherium paranense".