Again, let's look at the actual word. The first time I see it in the bible is in Job.
"Canst thou draw out leviathan with an hook? or his tongue with a cord which thou lettest down?"
The Hebrew definition for the word is this
"
leviathan, sea monster, dragon
- large aquatic animal
- perhaps the extinct dinosaur, plesiosaurus, exact meaning unknown"
Pagan mythos does not matter one whit. If I was going to consider a hunch or guess as to what the creature may be, I would first look to bible believers. One certainly would not look to unbelieving heathen myths to try and reinvent Scripture on that word!