A behemoth is something of enormous size or power. In the bible it is assumed to be a hippopotamus.
The behemoth is described in Job 40:15-24:
Look now at the behemoth,[a] which I made along with you; He eats grass like an ox. See now, his strength is in his hips, And his power is in his stomach muscles. He moves his tail like a cedar; The sinews of his thighs are tightly knit. His bones are like beams of bronze, His ribs like bars of iron. He is the first of the ways of God; Only He who made him can bring near His sword. Surely the mountains yield food for him, And all the beasts of the field play there. He lies under the lotus trees, In a covert of reeds and marsh. The lotus trees cover him with their shade; The willows by the brook surround him. Indeed the river may rage, Yet he is not disturbed; He is confident, though the Jordan gushes into his mouth, Though he takes it in his eyes, Or one pierces his nose with a snare.
Behemoth: A monster in Hebrew, Christian and Muslim traditions. Described as the male counterpart of the Leviathan. It is of enormous proportions, so vast that the desert of Dendain extends across its upper body; its bones are like brass; its huge jaws can consume thousands of acres of vegetation at one gulp and the river Jordan in one swallow. It was prophesized in Jewish tradition that at the Day of Judgement, the Leviathan and the Behemoth would be locked in combat to the death. The resulting bodies would be food for the "chosen", who had lived their lives in righteousness. In Christian tradition, Behemoth was regarded as a most powerful tempter that manifested in the shape of a grotesque elephant. His domain is that of the "delights of the belly" and he is charged by the Devil with tempting humans into the vice of overindulgence. In the Muslim tradition, Behemoth equates with the monstrous Bahamut.