Behemoth appears in a speech, not in a story. God is answering Job out of the whirlwind and the answer is a catalogue of creatures Job did not make and cannot control. Behemoth comes first and Leviathan follows, and the two are the climax of the argument: look at what is in the world, and consider whether you are in a position to audit its governance.
The description in the fortieth chapter of Job is physically specific. The creature eats grass like an ox, its strength is in its loins, it lies under the lotus in the marsh, and it is unmoved when the river rushes. Commentators reading for natural history identify it with the hippopotamus, and the marsh, the river and the grazing all fit. Commentators reading for myth point to the closing lines, which set the creature beyond human capture, and to the pairing with Leviathan, which is not an animal in any ordinary sense.
Both readings are old and neither excludes the other. Hebrew poetry can describe a real animal in terms borrowed from a mythological one, and the rhetorical force of the passage depends on the creature being both familiar and beyond reach. Later Jewish tradition took the second reading and made Behemoth the land counterpart in a scheme of three, which is where most later use of the name begins.