Wikipedia treats 'hippopotamus' as a genuine Latin word, derived from an ancient Greek one that's almost identical: hippopotamos. It has to be said though that the Latin for 'horse of the river' would be 'equus fluminis' – so strictly speaking the Latin word 'hippopotamus' is untranslatable.
In English, the plural of hippopotamus is 'hippopotamuses', but 'hippopotami' is also used.
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