Members of the order Perissodactyla, otherwise known as odd-toed ungulates, are mammals characterized by an odd number of toes and by hindgut fermentation with somewhat simple stomachs. Unlike the even-toed ungulates, they digest plant cellulose in their intestines rather than in one or more stomach chambers. The order includes three extant families: Equidae (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 as related families in the 19th century by the zoologist Richard Owen, who also coined the order name.