Perissodactyla

Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla (/pəˌrɪsoʊˈdæktɪlə/, from the Ancient Greek περισσός perissós, "uneven"; and δάκτυλος dáktylos, "finger, toe"), are hoofed animals—ungulates—which bear most of their weight on one (an odd number) of the five toes: the third toe. The non-weight-bearing toes are either present, absent, vestigial, or positioned posteriorly. By contrast, the even-toed ungulates bear most of their weight equally on two (an even number) of the five toes: their third and fourth toes. Another difference between the two is that odd-toed ungulates digest plant cellulose in their intestines rather than in one or more stomach chambers as the even-toed ungulates do.

The order includes about 17 species divided into three families: Equidae (horses, asses, and zebras), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs).

Despite their very different appearances, they were recognized as related families in the 19th century by the zoologist Richard Owen, who also coined the order name.