At the other extreme, an early member of the order, the prehistoric horse Eohippus, had a withers height of only 30 to. The largest odd-toed ungulates are rhinoceroses, and the extinct Paraceratherium, a hornless rhino from the Oligocene, is considered one of the largest land mammals of all time. 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. The order includes about 17 species divided into three families: Equidae (horses, asses, and zebras), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs). 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 even-toed ungulates, with the exception of Suina, do. By contrast, the even-toed ungulates bear most of their weight equally on four or two (an even number) of the five toes: their third and fourth toes. The non-weight-bearing toes are either present, absent, vestigial, or positioned posteriorly. Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla (, ), are animals-ungulates-who have reduced the weight-bearing toes to three (rhinoceroses and tapirs, with tapirs still using 4 toes on the front legs) or one (equines, third toe) of the five original toes. The white rhinoceros is the largest living perissodactyl

Western black rhinoceros anatomy