A large aquatic animal, perhaps the extinct dinosaur, plesiosaurus, the exact meaning is unknown. Some think this to be a crocodile but from the description in Job 41:1-41:34 this is patently absurd. It appears to be a large fire breathing animal of some sort. Just as the bomardier beetle has an explosion producing mechanism, so the great sea dragon may have an explosive producing mechanism to enable it to be a real fire breathing dragon.
H3882
לויתן
livyâthân
liv-yaw-thawn'
From H3867 ; a wreathed animal, that is, a serpent (especially the crocodile or some other large sea monster); figuratively the constellation of the dragon; also as a symbol of Babylon:—leviathan, mourning.