A sea creature no one can catch, described at more length than any other animal in the Bible.
God spends a whole chapter of Job on Leviathan, and the description is not restrained: rows of scales shut up tight as a seal, a back no harpoon troubles, smoke from the nostrils, flame from the mouth, and a wake left boiling behind it. Elsewhere it is the thing whose heads God crushed, and a creature made to play in the sea. Readings run from the crocodile through the whale to a mythic chaos-monster the surrounding cultures also knew, and the text supports the argument rather than settling it.
Disputed. Proposals include the crocodile, the whale, and a chaos-dragon drawn from the imagery of neighbouring cultures. Job 41 describes it in terms no living animal satisfies, which is part of why the question stays open.