The hippopotamus and rhinoceros are not related.
Both are large, stumpy-legged herbivores.
The hippopotamus is second only to the elephant in the category of “heaviest land mammal”; the rhinoceros is third.
The rhino is actually a very primitive land-dwelling mammal.
The hippo, in contrast, spends over half of its life in the water.
The hippopotamus is the third largest land animal in the world and can easily outrun a human.
And if you want to talk biology, the hippo’s closest living relative is the pig.