Job 40:20

The mountains bring him forth food - It cannot therefore be the hippopotamus, as he is seldom found far from the rivers where he has his chief residence.

Where all the beasts of the field play - He frequents those places where he can have most prey. He makes a mock of all the beasts of the field. They can neither resist his power, nor escape from his agility. All this answers to what we know of the mammoth, but not at all to the hippopotamus.

Surely the mountains bring him forth food - That is, though he lies commonly among the reeds and fens, and is in the water a considerable portion of his time, yet he also wanders to the mountains, and finds his food there. But the point of the remark here does not seem to be, that the mountains brought forth food for him, but that he gathered it "while all the wild beasts played around him, or sported in his very presence." It was remarkable that an animal so large and mighty, and armed with such a set of teeth, should not be carnivorous, and that the wild beasts on the mountains should continue their sports without danger or alarm in his very presence. This fact could be accounted for partly because the "motions" of the hippopotamus were so very slow and clumsy that the wild beasts had nothing to fear from him, and could easily escape from him if he were disposed to attack them, and partly from the fact that he seems to have "preferred" vegetable food. The hippopotamus is seldom carnivorous, except when driven by extreme hunger, and in no respect is he formed to be a beast of prey. In regard to "the fact" that the hippopotamus is sometimes found in mountainous or elevated places, see Bochart.

40:20 Mountains - Though he lives most in the water, yet he often fetches his food from the land, and from the mountains or hills, which are nigh the river Nile. Play - They not only feed securely, but sport themselves by him, being taught by experience that he is gentle and harmless.