This word is often used as a synonym for the whale or even the crocodile, but the only authentic description of Leviathan (in the Book of Job) shows he is a totally different type of creature.
He is a firebreathing sea monster, of such enormous size that the sea boils when he swims on the surface. He breathes smoke from his nostrils and flame from his mouth, which is rimmed with terrible heat. His skin is like a double coat of mail, covered with overlapping scales as big as shields on his back and as sharp and hard as broken pottery on his underparts. Swords, darts, javelins, harpoons and fishing spears simply bounce off this armoured hide.
Leviathan's character is ruthless and fearless. He has a heart as hard as a millstone and he is unperturbed by any attempts to catch him - not that any human huntsman is likely to make the attempt. The sight and sound of this enormous sea dragon, glaring around with fiery eyes as he churms through the waves, strikes terror into the heart of the mightiest warrior.