The hippopotamus (Hippopotamus amphibius), from the Greek (hippos meaning "horse" and potamus meaning "river") is a large, semi-aquatic, mostly plant-eating African mammal. The hippopotamus inhabits rivers and lakes in sub-Saharan Africa in groups of 5-30. During the day they remain cool, submerged in water or mud. At dusk they emerge to graze on grass and they return at dawn.