The closest living relatives of hippopotamuses are whales, dolphins, and porpoises.
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WHALES ARE DIVIDED INTO TWO MAIN GROUPS. There are two types of whales: the baleen whales and the toothed whales. Baleen whales, including humpbacks and blue whales, have fibrous ‘baleen’ plates in their mouths instead of teeth, which help them filter out and consume huge quantities of zooplankton: small prey including krill, fish or crustaceans called copepods. Whereas toothed whales, such as orcas, beluga and sperm whales, have teeth which enable them to feed on larger prey such as fish and squid. All dolphin families, including porpoises, are also classified as whales, as they are more related to their toothed counterparts.
Hippos’ closest living relatives are the aquatic mammals: whales, dolphins and porpoises.