The animal whose name translates as 'river horse' is the Hippopotamus. The name hippopotamus is derived from the ancient Greek word 'hippopotamos,' which means 'river horse.' Despite their name, hippos are not related to horses, and they are not closely related to any other living species today. Hippos are semi-aquatic mammals and are native to Africa. They spend most of their time in the water to keep their sensitive skin cool and hydrated.