It says "like" a cedar, not the "size" of a cedar.
Hippos have hair bristles on on their noses, ears and tails. And the shape of the bristles on their tails can have the appearance of the limbs on a lebanese cedar tree.
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The description in Job is a river animal.
It's a hippo.
NIV Job 40:15-24
15. "Look at the behemoth, which I made along with you and which feeds on grass like an ox.
16. What strength he has in his loins, what power in the muscles of his belly!
17. His tail sways like a cedar; the sinews of his thighs are close-knit.
18. His bones are tubes of bronze, his limbs like rods of iron.
19. He ranks first among the works of God, yet his Maker can approach him with his sword.
20. The hills bring him their produce, and all the wild animals play nearby.
21. Under the lotus plants he lies, hidden among the reeds in the marsh.
22. The lotuses conceal him in their shadow; the poplars by the stream surround him.
23. When the river rages, he is not alarmed; he is secure, though the Jordan should surge against his mouth.
24. Can anyone capture him by the eyes, or trap him and pierce his nose?
NIV Job 41:1-34
1. "Can you pull in the leviathan with a fishhook or tie down his tongue with a rope?
2. Can you put a cord through his nose or pierce his jaw with a hook?
3. Will he keep begging you for mercy? Will he speak to you with gentle words?
4. Will he make an agreement with you for you to take him as your slave for life?
5. Can you make a pet of him like a bird or put him on a leash for your girls?
6. Will traders barter for him? Will they divide him up among the merchants?
7. Can you fill his hide with harpoons or his head with fishing spears?
8. If you lay a hand on him, you will remember the struggle and never do it again!
9. Any hope of subduing him is false; the mere sight of him is overpowering.
10. No one is fierce enough to rouse him. Who then is able to stand against me?
11. Who has a claim against me that I must pay? Everything under heaven belongs to me.
12. "I will not fail to speak of his limbs, his strength and his graceful form.
13. Who can strip off his outer coat? Who would approach him with a bridle?
14. Who dares open the doors of his mouth, ringed about with his fearsome teeth?
15. His back has rows of shields tightly sealed together;
16. each is so close to the next that no air can pass between.
17. They are joined fast to one another; they cling together and cannot be parted.
18. His snorting throws out flashes of light; his eyes are like the rays of dawn.
19. Firebrands stream from his mouth; sparks of fire shoot out.
20. Smoke pours from his nostrils as from a boiling pot over a fire of reeds.
21. His breath sets coals ablaze, and flames dart from his mouth.
22. Strength resides in his neck; dismay goes before him.
23. The folds of his flesh are tightly joined; they are firm and immovable.
24. His chest is hard as rock, hard as a lower millstone.
25. When he rises up, the mighty are terrified; they retreat before his thrashing.
26. The sword that reaches him has no effect, nor does the spear or the dart or the javelin.
27. Iron he treats like straw and bronze like rotten wood.
28. Arrows do not make him flee; slingstones are like chaff to him.
29. A club seems to him but a piece of straw; he laughs at the rattling of the lance.
30. His undersides are jagged potsherds, leaving a trail in the mud like a threshing sledge.
31. He makes the depths churn like a boiling caldron and stirs up the sea like a pot of ointment.
32. Behind him he leaves a glistening wake; one would think the deep had white hair.
33. Nothing on earth is his equal-- a creature without fear.
34. He looks down on all that are haughty; he is king over all that are proud."
It's exactly what it sounds like -- a dragon. Leviathan was a local version of the age-old dragon myth, which dates back to at least 5000 BC with the Sumerians of Mesopotamia, the right time and place for the writers of the Bible to have heard and believed it and incorporated it into their writings.
Where is HIS fossil; since there appears to be NO creature like him to be found on Earth today.
Nor was there any such creature back in Biblical times. People have believed in a lot of myths through the ages.
(Even though in Job's time, men appear to have KNOWN about it.)
Men of ancient times also "knew" about griffons, flying horses, fairies, cyclopses, and countless various other mythical beings and monsters which we now know never actually existed.
People have "known about" dragons, garden fairies, Nessie, ghosts and Santa Claus. We even have photographs of garden fairies.