A lake monster is a lake-dwelling entity in 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").[1]