Sea serpent, in myths and legends from many parts of the world, large, snakelike sea animal. Sea monsters of various description appear in ancient myths of the Middle East. Sightings of sea serpents, especially in the North Atlantic, have been common but have never been confirmed by material evidence. It is thought that what have appeared to be sea serpents have actually been giant squid, masses of seaweed, schools of porpoises, and other natural phenomena. The Loch Ness Monster is a similar freshwater creature thought to inhabit a lake in northern Scotland.