Commentators on the Bible say that leviathan is a whale or a crocodile. Below I point out why it is neither, but probably a fire breathing dragon as many stories indicate.
Job 41:1 Canst thou draw out leviathan with an hook? or his tongue with a cord which thou lettest down? Defenders Study Bible notes: Leviathan was evidently the greatest of the marine reptiles, or dinosaurs, something like a plesiosaur, perhaps, although modern commentators tend to call it a crocodile. Isaiah says that leviathan was “the dragon in the sea” (Isaiah 27:1), and the psalmist said that leviathan “played” in the “great and wide sea” (Psalm 104:25,26). (Personally I don’t agree that it is a plesiosaur, but probably an extinct kind of dinosaur.)
2 Canst thou put a hook into his nose? or bore his jaw through with a thorn?
3 Will he make many supplications unto thee? will he speak soft words unto thee?
4 Will he make a covenant with thee? wilt thou take him for a servant forever?
5 Wilt thou play with him as with a bird? or wilt thou bind him for thy maidens?
6 Shall the companions make a banquet of him? shall they part him among the merchants?
7 Canst thou fill his skin with barbed iron? or his head with fish spears?
8 Lay thine hand upon him, remember the battle, do no more.
9 Behold, the hope of him is in vain: shall not one be cast down even at the sight of him?
10 None is so fierce that dare stir him up: who then is able to stand before me? Defenders Study Bible notes: Although no man could overcome the leviathan, God could do so, just as He could overcome the behemoth.
11 Who hath prevented me, that I should repay him? whatsoever is under the whole heaven is mine.
12 I will not conceal his parts, nor his power, nor his comely proportion.
13 Who can discover the face of his garment? of who can come to him with his double bridal?
14 Who can open the doors of his face? his teeth are terrible round about. (Whales have terrible teeth?)
15 His scales are his pride, shut up together as with a close seal. (Whales have scales?)
16 One is so near to another, that no air can come between them.
17 They are joined one to another, they stick together, that they can not be sundered.
18 By his neesings a light doth shine, and his eyelids are like the eyelids of the morning.
19 Out of his mouth go burning lamps, and sparks of fire leap out.
20 Out of his nostrils goeth smoke, as out of a seething pot or cauldron.
21 His breath kindleth coals, and a flame goeth out of his mouth. Defenders Study Bible notes: Whatever the leviathan was, it was not a crocodile. Many of the dragon legends indicate they could breath fire, and there are indications that at least certain dinosaurs may have been able to produce and expel combustible gases which, upon coming in contact with oxygen, could have ignited.
22 In his neck remaineth strength, and sorrow is turned into joy for him. (Please show me the neck on a whale.)
23 The flakes of his flesh are joined together: they are firm in themselves; they cannot be moved.
24 His heart is as firm as a stone; yea, as hard as a piece of the nether millstone.
25 When he raiseth up himself, the mighty are afraid: by reason of breakings they purify themselves. (The mighty fear when the crocodile rises up?)
26 The sword of him that layeth at him cannot hold: the spear, the dart, nor the habergeon.
27 He esteemeth iron as straw, and brass as rotten wood.
28 The arrow cannot make him flee: sling-stones are turned with him into stubble.
29 Darts are counted as stubble: he laugheth at the shaking of a spear.
30 Sharp stones are under him: he spreadeth sharp pointed things upon the mire.
31 He maketh the deep to boil like a pot: he maketh the sea like a pot of ointment.
32 He maketh a path to shine after him; one would think the deep to be hoary.
33 Upon earth there is not his like, who is made without fear.
34 He beholdeth all high things: he is king over all the children of pride. (Here God is comparing leviathan to Satan.)
This is Ken Ham’s idea of what leviathan looked like. We can’t make a picture that is %100 accurate, but I’d say that this is pretty close.