The word hippopotamus is derived from the Greek words hippos (horse) and potamos (river). River horse. Hippopotamus.
Here's a new noun I just made up: hippobottomus. From the Greek hippos (horse) and the common English bottom (buttocks). Hippobottomus. Horse's ass.
I also made up the verb hippopotamize, but I don't yet know what it means. To transform something into a hippopotamus? To dunk a horse in the river? To turn a river into a horse, the way Liv Tyler did in The Lord of the Rings: The Fellowship of the Ring?