In folklore, a lake monster is a fresh-water dwelling entity, often monstrous. A very well-known example is the Loch Ness Monster. Lake monsters' depictions are often similar to some sea monsters.
Well-known lake monsters include:

Nessie, in Loch Ness, Scotland
Morag, in Loch Morar, Scotland
Lagarfljót Worm, in Lagarfljót, Iceland
Ogopogo, in Okanagan Lake, Canada
Lariosauro, in Lake Como, Italy
Champ, in Lake Champlain, Canada and US
Memphre, in Lake Memphremagog, Canada and US
Nahuelito, in Nahuel Huapi Lake, Argentina
Muyso or Monster of Lake Tota, in Lake Tota, Colombia
Van Gölü Canavarı or Lake Van Monster, in Lake Van, Turkey
Inkanyamba, in Howick Falls, South Africa
Tahoe Tessie, in Lake Tahoe, US
Pepie, in Lake Pepin, US
Flathead Lake Monster, Flathead Lake, Montana, US