The horse relatives, rhinoceroses, and tapirs are hoofed animals (ungulates) that share the common feature of an odd number of toes. Although their ancient ancestors had five toes, the rhinoceros and the tapir now have three toes, and the horse relatives have a single toe covered in a hoof. The main toe that bears the weight of the animal corresponds to the middle toe of their ancestors. The remaining toes either bear less weight, or have disappeared altogether.
These animals differ from the other hoofed ungulates (giraffes, swine, ruminants and hippos) in the arrangement of their digestive system. Past the small intestine, there is a large pouch (cecum) where their plant food is stored and digested by bacteria. This arrangement is called “hind gut fermentation”.
Today, only 7 species of horse relatives exist: the horse, 3 species of zebras and 3 species of wild asses. Only 5 species of rhinos exist, 2 in Africa and 2 in Asia. Likewise, only 5 species of tapirs exist, 4 in South and Central America and 1 in Asia.