The word "hippopotamus" comes from the ancient Greek , "hippopotamos", from , "hippos", "horse", and , "potamos", which means "river". So, "hippopotamus" means "horse of the river". More than one hippopotamus are called "hippopotami", and 'hippopotamuses', or 'hippos' is also used. Hippopotamusi are social. About 30 hippos live together in groups. A male hippopotamus is known as a bull. A female hippopotamus is called a cow, and a baby hippo is called a calf. The species is also known as the common hippopotamus or the Nile hippopotamus.