A lake monster is a lake-dwelling creature in myth and folklore. The most famous example is the Loch Ness Monster. Depictions of lake monsters are often similar to those of sea monsters.
In the Motif-Index of Folk-Literature, entities classified as "lake monsters", such as the Scottish Loch Ness Monster, the American Chessie, and the Swedish Storsjöodjuret fall under B11.3.1.1. ("dragon lives in lake").
Storsjön (Swedish pronunciation: [ˈstûːrɧøːn], lit. 'The Great Lake') is the fifth largest lake in Sweden, with an area of 464 km (179 sq mi) and a greatest depth of 74 m (243 ft). It is the largest lake in central Sweden, located in the province of Jämtland in modern Jämtland County. From Storsjön runs the river Indalsälven and the lake contains the major island Frösön. The city of Östersund is located on the east shore of the lake, opposite Frösön.
Storsjön is said to be the home of Storsjöodjuret, a cryptid lake monster not unlike the Loch Ness Monster, and every now and then there are new reports of people having spotted it. Descriptions of the creature have varied over the years. Some have described it as being serpentine in appearance, with multiple humps, a feline or canine-like head and grayish skin. Others have claimed that the creature is short in stature and morbidly obese, with a roundish skull.
The Lernaean Hydra or Hydra of Lerna (Ancient Greek: Λερναῖα ὕδρα, romanized: Lernaîa Húdrā), more often known simply as the Hydra, is a serpentine lake monster in Greek mythology and Roman mythology. Its lair was the lake of Lerna in the Argolid, which was also the site of the myth of the Danaïdes. Lerna was reputed to be an entrance to the Underworld, and archaeology has established it as a sacred site older than Mycenaean Argos. In the canonical Hydra myth, the monster is killed by Heracles (Hercules) as the second of his Twelve Labors.
According to Hesiod, the Hydra was the offspring of Typhon and Echidna. It had poisonous breath and blood so virulent that even its scent was deadly. The Hydra possessed many heads, the exact number of which varies according to the source. Later versions of the Hydra story add a regeneration feature to the monster: for every head chopped off, the Hydra would regrow two heads. Heracles required the assistance of his nephew Iolaus to cut off all of the monster's heads and burn the neck using a sword and fire.