The hippopotamus [Hippopotamus amphibious], or hippo, from the ancient Greek for “river horse”, is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae [the other is the pygmy hippopotamus]. After the elephant and rhinoceros, the hippopotamus is the third largest type of land mammal and the heaviest extant artiodactyl.
Despite their physical resemblance to pigs and other terrestrial even-toed ungulates, their closest living relatives are cetaceans [whales, porpoises, etc.] from which they diverged about 55 million years ago. The common ancestor of whales and hippos split from other even-toed ungulates around 60 million years ago. The earliest known hippopotamus fossils, belonging to the genus Kenyapotamus in Africa, date to around 16 million years ago.
The hippopotamus is semi-aquatic, inhabiting rivers, lakes and mangrove swamps, where territorial bulls preside over a stretch of river and groups of 5 to 30 females and young. During the day, they remain cool by staying in the water or mud; reproduction and childbirth both occur in water.