From Longman Dictionary of Contemporary Englishhippopotamuship‧po‧pot‧a‧mus /ˌhɪpəˈpɒtəməs $ -ˈpɑː-/ noun (plural hippopotamuses or hippopotami /-maɪ-/) [countable] 
HBAa large grey African animal with a big head and mouth that lives near water → rhinoceros
HBAa large grey African animal with a big head and mouth that lives near water → rhinocerosExamples from the Corpus
hippopotamus• A delightful unguent jar from Mostagedda carved from ivory in the form of a hippopotamus argues that the animal attracted favourable attention.• They don't know the maximum untaxed income, but they do know George Washington had hippopotamus teeth.• Nevertheless, the marriage of hippopotamus with whale is clear.• It includes every kind of artistic endeavour - from conventional watercolours to a pink hippopotamus made out of an old car.• Rather more realistically, perhaps, the behemoth has been latterly identified as either the hippopotamus or rhinoceros.• This seems hard on the hippopotamus.• The hippopotamus is a vegetarian and looks like a wall.Origin hippopotamus (1500-1600) Latin Greek, from hippos “horse” + potamos “river” 