The land-beast God shows Job just before Leviathan, the sea-beast. Its name is simply the Hebrew word for "beast" in a form that means something like "the beast of beasts". A hippopotamus is the most popular identification, but the poem is less interested in which animal it is than in the fact that only God can master it.
One passage, ten verses (Job 40:15–24), in God's second speech to Job.
Behold now behemoth, which I made with thee; he eateth grass as an ox.Job 40:15 — made alongside Job, and a grass-eater
He moveth his tail like a cedar: the sinews of his stones are wrapped together.Job 40:17 — the verse every identification has to explain
He is the chief of the ways of God: he that made him can make his sword to approach unto him.Job 40:19 — the first of God's works, and only its Maker can approach it
Behold, he drinketh up a river, and hasteth not: he trusteth that he can draw up Jordan into his mouth.Job 40:23 — unhurried by a flood
The point of the speech is not natural history. Job has demanded that God answer him, and God answers by showing him two creatures Job cannot control. If Job cannot master what God made, he is in no position to sit in judgment on how God runs the world.
Behemoth is paired with Leviathan, which follows straight after (Job 41). One rules the land and one the sea, so between them they stand for everything wild and untameable in creation.
Graded by how sure it is possible to be.
The common mistake is to treat the passage as a field guide and to stake everything on getting the species right. The writer is using the most powerful animals his readers knew, heightened by poetry, to say one thing: you did not make this, and you cannot tame it.
The opposite mistake is to call Behemoth pure myth. The text says plainly that God made it "with thee", alongside Job, as a creature. Whatever it is, it is something God made, not a rival god.
The "tail like a cedar" is still the live question. Is it a euphemism, a poet's exaggeration of a real animal, or a sign that Behemoth was always meant to be more than any one animal?