The word hippopotamus comes directly from its Latin cognate hippopotamus, which itself underwent very little alteration as we trace it back to the Ancient Greek word hippopotamos. This word is a combination of two previous Greek words: hippo, which meant "horse" and potamos, which meant "river". Therefore, a hippopotamus is the "horse of the river", which kind of makes sense. But technically the modern abbreviation hippo doesn't refer to the fat animal, etymologically speaking. Anyway, hippo comes from the Proto-Hellenic ikkwos, from Proto-Indo-European hekus, "swift". Potamus officially has an uncertain etymology but may come from the earlier word pipto, which meant "to fall" (since a river falls) and that would be from the Proto-Indo-European word peth, meaning "to fly", which also makes sense. Yay, fun!
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.