“What was the leviathan?”……There are several references to “leviathan” in the Old Testament, but their meaning is sometimes poetic and often obscure. In one instance, the Hebrew word for “leviathan” is used as a synonym for earthly kings and princes, possibly Pharaoh

Isaiah 27:1

In that day the Lord will take his terrible, swift sword and punish Leviathan the swiftly moving serpent, the coiling, writhing serpent. He will kill the dragon of the sea.

and in another instance, it is translated “mourning (KJV) or cursing(NLT)”

Job 3:8

Let those who are experts at cursing —  whose cursing could rouse Leviatha —…curse that day

(The identification of Leviathan is disputed, ranging from an earthly creature to a mythical sea monster in ancient literature.)

The verses that actually refer to leviathan as a creature don’t provide enough information to make a determination one way or the other regarding what type of creature leviathan is. Some commentators believe leviathan is a crocodile because of the references to its scales

Job 41:15

Its scales are like rows of shields tightly sealed together.

and its use as a food source

Psalm 74:14

You crushed the heads of Leviathan and let the desert animals eat him.

Some commentators believe that leviathan is a large sea reptile (not a whale), possibly even a species of dinosaur.……The fact that leviathan is a creature is beyond doubt and, as such, it is under God’s sovereign control. God uses the leviathan’s strength and power to illustrate to Job his weakness and frailty. God asks Job to consider how powerless he is against even the sea creatures God has created and to understand his position in the universe. God points out to Job that he could never pull the leviathan out of the water with a hook, which would seem to indicate that leviathan could be a whale. Whatever its nature, leviathan is large and powerful enough to be subject only to the sovereign control of the Lord.……Leviathan was a real creature, unlike some of the mythological representations of great sea creatures that do battle with the gods. Leviathan is no myth, but rather a creature of the sea, whether a fish, a sea dragon, a mammal such as a whale, or a reptile such as a crocodile. It is also possible that the leviathan, whatever it was, is now extinct.