Which animal's name comes from the Greek for "river horse"?

The Greeks called it hippopotamus, from hippos for horse and potamos for river, because they met the animal in the Nile and reached for the largest familiar land mammal to describe it. The name stuck across European languages even after the comparison turned out to be badly wrong. Hippos are not related to horses in any meaningful way. Genetic work places their closest living relatives among the cetaceans, the whales and dolphins, with a shared ancestor that lived roughly 55 million years ago. That relationship explains features which look strange on a land animal: hippos give birth and nurse underwater, they have no sebaceous glands or true sweat glands, and they communicate with clicks and calls that carry underwater. What looks like a swimming horse is closer to a whale that never fully left.

Hippos cannot actually swim, and move underwater by pushing off the bottom.