Some believe that Psalm 74 contains the oldest biblical reference to the leviathan. A fragment of an ancient creation story begins at verse 12, with God defeating the primeval leviathan in verse 14. The leviathan, a sea monster, appears to have been an adversary of God. E. A. Wallace Budge (The Gods of the Egyptians) points out that the Psalm suggests that the leviathan had several heads. He says that all the available evidence is that the leviathan is a Hebrew conception that originated in Babylon.
The following are all the Biblical references to this creature. The Biblical Hebrew literally means 'a wreathed animal' i.e. a serpent.
1Canst thou draw out leviathan with an hook? or his tongue with a cord which thou lettest down?
14Thou brakest the heads of leviathan in pieces, and gavest him to be meat to the people inhabiting the wilderness.
26There go the ships: there is that leviathan, whom thou hast made to play therein.
1In that day the LORD with his sore and great and strong sword shall punish leviathan the piercing serpent, even leviathan that crooked serpent; and he shall slay the dragon that is in the sea.
Whatever Leviathan was, this creature had some kind of an estuarine or marine habitat for its home, and it was certainly a rather large and 'terrible' thing to behold - what some would correlate with a dinosaur, although this is not conclusive, as it could have been a fish (unlikely) or a mammal. The idea that this creature was an adversary of God is supported by the Isaiah verse as well - or at least it is seeking to demonstrate that God's great power was even greater than this mighty creature, and so He could easily overcome it.