Late-night National Geographic discovery: hippos, whales, and dolphins are related. No wonder I've always had a thing for those three animals. And otters. I love otters.
Evidently hippos click and call underwater like dolphins & whales. Scientists tested their DNA and discovered that they all probably evolved from a hippo-like animal. Wild!
And yes, I know that the channel now goes by "NAT GEO" but I refuse to call it "NAT GEO". That's like giving in and calling Burger King "BK". I mean, I've certainly referred to McDonald's as MickeyD's, but they aren't about to change their branding just because that's catchy. (I hope.)
And in case you're sitting there wondering what the first hippowholphin looked like, Wikipedia has more info.
As indicated by the name, ancient Greeks considered the hippopotamus to be related to the horse. Until 1985, naturalists grouped hippos with pigs, based on molar patterns. Evidence, first from blood proteins, then from molecular systematics and DNA  and the fossil record, show that their closest living relatives are cetaceans—whales, porpoises and the like. Hippopotamuses have more in common with whales than they do with other Artiodactyls (even-toed ungulates), such as pigs, because the common ancestor of hippos and whales branched-off from ruminants and the rest of the even-toed ungulates. Thus, hippos are more closely related to whales than to other members of Artiodactyla. While cetaceans and hippos are each other's closest living relatives, their lineages split soon after their divergence from the rest of the even-toed ungulates.