The leviathan is a huge and fierce water monster, mentioned in several books of the Bible. Mention occurs in the books of Job, Psalms, and Isaiah, and there are several traditional Jewish prayers that also include the leviathan. Christian evaluation of the creature tends to associate the leviathan with the devil or demons, where the Old Testament view often looks at the creature as a natural creature.
Although in pictures, the leviathan is often depicted as a large water dragon, sometimes with multiple heads, many feel strongly that the word was alternate for crocodile, a creature that would have proliferated along the Nile. On the other hand, Job describes the beast as capable of breathing fire, which suggest more dragonlike characteristics. There may be some connection of the creature to Canaanite beliefs, or Ugaritic myths. A creature called Lotan dwells in the water and resembles a seven-headed dragon. In Ugaritic mythology, Baal Hadad fights with and destroys Lotan.
Leviathan is often intimately connected to the idea of chaos and destruction. In Ugaritic mythology, Lotan summons floods and tidal waves. The biblical equivalent is sometimes viewed as the opposite of creation, and is chaotic and destructive in nature. Early Christian writings take this one step further, especially the writing of Thomas Aquinas who associates the creature with envy and also calls it a demon. In constructing the punishments of hell, leviathan is called to punish the sinners who committed the deadly sin of envy and died without confession and absolution.
From a purely natural world standpoint, connecting the sea monster to the crocodile makes a lot of sense. It could certainly instantly cause death and was a thing to be greatly feared. When the creature is thought of as a simple beast, which it is in at least one Jewish prayer, and in one Midrash, a retelling — often highly metaphorical — of different instances in the Bible, the beast is God’s creation but so fearsome that God destroys the female so that the creature cannot mate.
On the other hand, a prayer mentions: “the sukkah of the skin of Leviathan,” and is said during the celebration of Sukkot or Festival of Booths. A sukkah is a temporary structure, like a tent, that Jews who celebrate Sukkot may choose to live in during the Festival. Part of the Sukkah must be made of natural materials, and crocodile skin might have been used in the past.
In Modern Hebrew, leviathan can be translated as whale, so there is also some context for considering a large sea mammal as this creature. This concept though, in most fantasy tellings is mainly lost in preference of depicting the animal as a water dragon, sometimes capable of flight. In the Final Fantasy series of video games, characters can summon this creature, which looks like a large blue dragon that rains watery destruction on monsters.