Sea-serpent: a sea-monster of serpentine form and great length, frequently reported to have been seen at sea. As history evolved and knowledge of dinosaurs and their aquatic cousin, plesiosaurs became more pronounced, the sea serpents began to take the shape of the long extinct reptiles. However, in the 18th century, the sea-serpent was more akin to a giant snake.
Early skeptics dismissed sea-serpents as tricks of light and lumps of sargassum (sea weed). Others claimed the sighting to be whales, schools of fish, dolphins, and seals. As with the kraken, the sea-serpent may actually be a living deep-sea creature, however smaller than the accounts given by sailors of old. The oarfish fits the role of the original sea-serpent quite nicely. Reaching lengths of 8 meters (26 feet)* and weighing as much as 270 kg (600 pounds) could give superstitious pirate a bad night's sleep.
*Unverified reports have reached lengths of 17 meters, about 66 feet.