The Leviathan is an tremendously huge sea monster that was mentioned in Old Testament of the Bible.
His name is said to mean "Twisted, or coiled" in Hebrew. Apparently though, in Modern Hebrew the word simply means "whale".
Job chapter 41 describes him is great detail. Judging by the way he is described, Leviathan is believed to be a fire-breathing sea dragon.
Job chapter 41:1-4 and 15-21
1 CANST thou draw out leviathan with an hook? or his tongue with a cord that thou lettest down?
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 though take him for a servant forever?
15 His scales are his pride, shut up together as with a close seal.
16 One is so near to another, that no air can come between them.
17 They are joined one to another, that they stick together, that they cannot be sundered.
18 By his neesings a light doth shine, and his eyes 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 caldron.
21 His breath kindleth coals, and a flame goeth out of his mouth.