Perissodactyls, otherwise known as odd-toed ungulates, compose an order of mammals characterized by an odd number of toes and being hindgut fermenters with somewhat simple stomachs. As large grazers they digest plant cellulose in their intestines rather than in one or more stomach chambers, unlike the even-toed ungulates. The order includes three extant families: Equidae, including horses, donkeys, and zebras, Rhinocerotidae, rhinos and Tapiridae, tapirs, with a total of about 17 species. In spite of their very different appearance they were recognized by the zoologist Richard Owen, who also coined the term, as related families, in the 19th century.