Adam Clarke on 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.