Lake monster or loch monster is a term referring to purported fresh-water dwelling megafauna appearing in mythology, rumor, or local folklore, but whose existence lacks scientific support. A well known example is the Loch Ness Monster. Lake monsters' depictions are often similar to some sea serpents. They are principally the subject of investigations by followers of the discipline of cryptozoology.
Many skeptics consider lake monsters to be purely exaggerations or misinterpretations of known and natural phenomena, or else fabrications and hoaxes. Most lake monsters have no evidence besides alleged sightings and controversial photographs and a large portion are generally believed not to exist by conventional zoology and allied sciences. Misidentified sightings of seals, otters, deer, diving water birds, large fish such as giant sturgeons, logs, mirages, seiches, light distortion, crossing boat wakes, or unusual wave patterns have all been proposed to explain specific reports. Social scientists point out that descriptions of these creatures vary over time with the values and mood of the local cultures, following the pattern of folk beliefs and not what would be expected if the reports were of actual encounters with real animals.
According to the Swedish naturalist and author Bengt Sjögren (1980), the present day belief in lake-monsters is associated with the legends of kelpies. Sjögren claims that the accounts of lake-monsters have changed during history. Older reports often talk about horse-like appearances, but more modern reports often have more reptile and dinosaur-like-appearances, and Sjögren concludes that the legends of kelpies evolved into the present day legends of lake-monsters where the monsters changed the appearance since the discovery of dinosaurs and giant aquatic reptiles from the horse-like water-kelpie to a dinosaur-like reptile, often a plesiosaur.
Other widely varied theories have been presented by believers, including unknown species of giant freshwater eels or surviving aquatic, prehistoric reptiles, such as plesiosaurs. One theory holds that the monsters that are sighted are the occasional full-grown form of an amphibian species that generally stays juvenile all its life like the axolotl. Cryptozoologist Bernard Heuvelmans held throughout his life that plesiosaur-type sighting were actually an unknown species of long-necked seal.
In many of these areas, especially around Loch Ness, Lake Champlain and the Okanagan Valley, these lake monsters have become important tourist draws.