You know, that's essentially what dolphins and whales are. Oh, granted, they aren't actually hippos or mackerel, but they evolved from hippo-like ungulates millions of years ago (back when leptis bounced around), and evolutionary pressure have given them similar body plans to fish like mackerel.
(Yeah, I know you were making a joke, but that doesn't mean you were wrong.)
You know, that's essentially what dolphins and whales are. Oh, granted, they aren't actually hippos
