Herodotus also has something to say about hippos:
The hippopotamus is held sacred in the district of Papremis, but not elsewhere. This animal has four legs, cloven hoofs like an ox, a snub nose, a horse’s mane and tail, conspicuous tusks, a voice like a horse’s neigh, and is about the size of a very large ox. Its hide is so thick and tough, that when dried it can be made into spear shafts.(Herodotus, Book II.71)
The notes on this passage in my penguin translation say:
The description of the hippopotamus represents the nadir of Herodotean zoology. Clearly he saw neither the real thing nor pictures of it, and his description is based purely on its Greek name, which means ‘river-horse’.
If only Herodotus had seen this mosaic he might have been able to give a better description- though I’m not sure hippos are supposed to have such evil grins.