Artiodactyla is an order of mammals (class Mammalia) that includes the hippos, camels, deer, giraffe, pronghorns, and the buffalo. All have an even number of toes (2 or 4); that is, they have cloven hooves, a specialization that originally evolved for fast running. All are herbivorous. They are mostly of Old World origin and range in size from the 3.6-kilogram (8-pound) mouse deer to the 4.5-ton hippopotamus. Among the artiodactyls are to be found some of the most important domestic animals such as pigs, cattle, sheep, and goats. See also ungulate.