In the Bible Leviathan is a great sea-creature named in several books (Job, Psalms, Isaiah); the longest description is God's speech about Leviathan in Job 41.
Job 41:1-2 in the King James Version reads:
1 Canst thou draw out leviathan with an hook? or his tongue with a cord which thou lettest down? 2 Canst thou put an hook into his nose? or bore his jaw through with a thorn?
These verses open Job 41, the chapter God devotes to describing Leviathan. Read it in context at Job 41:1-2, or the full chapter at Job 41. Encyclopedia entry: en.wikipedia.org/wiki/Leviathan.