Etymology
From Latin and New Latin hippopotamus, from Ancient Greek ἱπποπόταμος (hippopótamos), from ἵππος (híppos, “horse”) (English hippo-) + ποταμός (potamós, “river”). First attested in the 1560s.
Displaced native Old English sǣhenġest, and largely displaced English nicker (from native Old English nicor) and river horse.