Behold now behemoth, which I made with thee; he eateth grass as an ox. Lo now, his strength is in his loins, and his force is in the navel of his belly. He moveth his tail like a cedar: the sinews of his stones are wrapped together. His bones are as strong pieces of brass; his bones are like bars of iron. He is the chief of the ways of God: he that made him can make his sword to approach unto him. Surely the mountains bring him forth food, where all the beasts of the field play. He lieth under the shady trees, in the covert of the reed, and fens. The shady trees cover him with their shadow; the willows of the brook compass him about. Behold, he drinketh up a river, and hasteth not: he trusteth that he can draw up Jordan into his mouth. He taketh it with his eyes: his nose pierceth through snares.
(Job 40:15-24 KJV)
The Hebrew word used for Behemoth in the Book of Job is transliterated into English as Bĕhemowth (בַּהֲמוֹת). It is speculated that Behemoth was a now extinct sauropod sauropod , perhaps a Diplodocus or Brachiosaurus. Some scholars have interpreted Behemoth to be an elephant or hippopotamus, but from the detailed description in Job 40:15-24, this is obviously a ridiculous conclusion. Behemoth is a masculine noun, but a singular noun of Egyptian derivation.
Interestingly, the Hebrew word Bĕhemah (בְּהֵמָה), appears throughout the Tanakh in reference to God’s creation of beasts. Bĕhemah is a feminine noun, but it is plural.
The beasts [Bĕhemah] of the field cry also unto thee: for the rivers of waters are dried up, and the fire hath devoured the pastures of the wilderness.
(Joel 1:20 KJV)
Behemoth is a beast mentioned in the Book of Job and the Book of Joel. Metaphorically, the name has come to be used for any extremely large or powerful entity.
In Jewish aprocrypha and pseudepigrapha such as the Book of Enoch, Behemoth is the primal unconquerable monster of the land, as Leviathan is the primal monster of the waters of the sea and Ziz (a cockatrice or griffin) the primordial monster of the sky. In Book of Enoch, Leviathan lives in the Abyss, while Behemoth the land-monster lives in an invisible desert east of the garden of Eden. A Jewish rabbinic legend describes a great battle which will take place between them at the end of time. It says, “they will interlock with one another and engage in combat, with his horns the Behemoth will gore with strength, the fish [Leviathan] will leap to meet him with his fins, with power. Their Creator will approach them with his mighty sword [and slay them both].” Then, “from the beautiful skin of the Leviathan, God will construct canopies to shelter the righteous, who will eat the meat of the Behemoth and the Leviathan amid great joy and merriment.” (Artscroll Siddur, p. 719).
The Young Earth Creationist opinion is that Leviathan and Behemoth are names given to dinosaurs which lived in Biblical times.
Since the 17th century there have been many attempts to identify Behemoth. Some scholars have seen him as a real creature, usually the hippopotamus, although occasionally as the elephant, crocodile, water buffalo or for some creationists, a dinosaur. The reference to Behemoth’s “tail” that “moves like a cedar” (Job 40:17), is a problem for those who dismiss Behemoth as a hippopotamus, elephant, crocodile or water buffalo, since it cannot easily be identified with the tail of any of these animals or their distant ancestors.
You can draw any conclusion you want from this article, but I believe entirely that Behemoth was a now extinct sauropod dinosaur.