A legendary serpent-like sea monster. It seems that they never attacked boats, but they were fearful to behold, and dreaded by sailors. They were seen only in fine or calm weather, and traveled on the surface of the water at great speed.
Some believed that a sea serpent caused the tides to rise and fall:
Away far out in the sea, near the edge of the world, there lived a monstrous sea-serpent that took about six hours to draw in his breath, and six hours to let it out, thus causing the rise and fall of the water.
- Teit, James A. (1918). "Water-beings in Shetlandic Folk-Lore, as remembered by Shetlanders in British Columbia." JAF 31:180-201, p. 198.