I was intrigued on seeing your question. I didn't know tha answer. So I did a websearh. And I am astonished to find that closest relatives hippos are the whales! . 'Evolution The most recent theory into the origins of hippopotamidae suggests that hippos and whales shared a common semi-aquatic ancestor that branched off from other Artiodactyls around 60 million years ago.[9][8] This hypothesized ancestral group likely split into two branches around 54 million years ago.[7] One branch would evolve into cetaceans, possibly beginning with the proto-whale Pakicetus from 52 million years ago with other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.[11] Anthracotherium magnus and Elomeryx armatus, two anthracotheres from the Oligocene, already bore similarities to the modern hippopotamus. The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of whom in the Late Eocene would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene without leaving any descendants.[9] A rough evolutionary lineage can be traced, however, from Eocene and Oligocene species: Anthracotherium and Elomeryx to the Miocene Merycopotamus and Libycosaurus and the very latest anthracotheres in the Pliocene.[12] Merycopotamus, Libycosaurus and all hippopotamids can be considered to form a clade, with Libycosaurus being more closely related to hippos. Their common ancestor would have lived in the Miocene, about 20 million years ago.' . http://en.wikipedia.org/wiki/Hippopotamus .

a hippo...

They ate their own species and very distantly related to whales. http://www.bbc.com/earth/story/20150116-the-diet-secrets-of-hippos-herbivore-or-cannibal

Just a guess! MOUSE?