In the Bible, at the outset of creation, Jehovah is said to have destroyed Leviathan, the primeval (sea) dragon. The story probably goes back to a Babylonian version in which Marduk defeats the sea monster Tiamat.
In galactic lore, there are stories of an alien saurian race of giant sea serpents. These, too, are oftern referred to as Leviathans. They would have extreme psychic powers and would be used here on Earth to influence the Collective Unconscious, as part of a psychological warfare scheme.
Some sources claim the Monster of Loch Ness would be such a Leviathan.
[I have not been able to verify how reliable these stories are, and personally have some reservations. Still, cryptozoologists at least mention an increase in the number of sightings of giant sea serpents, so their existence seems to be getting more confirmation].