A class of mammal which might be mistaken for monsters would be the Sirenias. This group includes dugongs and sea cows. The most familiar of these animals is the manatee that swims in the rivers and along the coast of Florida.

Manatee's have two forward flippers and a flat tail, like the seal. Unlike the seal it spends almost all of it's time in the water. Also unlike the seal it feeds on aquatic vegetation. Manatee's grow up to 15 feet long and weigh 3500 pounds. Could a relative of the manatee be a lake monster? Maybe. Many believe that the manatee and it's cousin, the sea cow, may have inspired the tales of mermaids, though it would take a particularly lonesome sailor to mistake the face of one of these creatures for a beautiful woman.