Albert Barnes Commentary Job 40:20

"Surely the mountains bring him forth food, Where all the beasts of the field do play." — Job 40:20 (ASV)

Surely the mountains bring him forth food - That is, though he commonly lies among the reeds and marshes, 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.

Regarding the fact that the hippopotamus is sometimes found in mountainous or elevated places, see Bochart.