Artiodactyla is an order of mammals (class Mammalia) that includes the hippos, camels, deer, giraffe, pronghorns, and 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-kg (8-lb) 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.
Cetaceans (whales, dolphins, and porpoises) evolved from even-toed ungulates, and are therefore often classified under the same taxonomic branch due to the fact that a species cannot outgrow its evolutionary ancestry; some modern taxonomists combine the two under the name Cetartiodactyla, while others opt to include cetaceans in the already-existing Artiodactyla.