Leviathan is an huge aquatic creature described in the Old Testament. It was a kind of mythological creature, but sometimes the "monster" is said to be a large dolphin or whale. The main lesson of unbeatable Leviathan's appearance in the Bible is that people can not judge God’s activity relative to the earth inhabitants created by Him.

This can be interesting: Today the word "Leviathan" is a synonym for any enormous sea creature or monster.

More Info: en.wikipedia.org