Look at all the lake monsters on this map! Are there really that many monsters in American lakes? Maybe not, but there are at least that many lake monster legends. The full map at Atlas Obscura has a legend of its own, to explain the different types of monsters. There are serpents (like Nessie), giant fish monsters, monstrous turtles, and even a cephalopod or two. A few of the monsters have their story posted, like the Alkali Lake Monster.
A 1923 eyewitness account of the monster of Alkali Lake in Nebraska reported that the single-horned alligator-esque creature let out a "dreadful roar" from the water. Described as 40 feet long with a horn like a rhinoceros, as well as an atrocious smell that is near fatal in itself, it's apparently a very hungry creature and devours anything in its path. Mostly this has been livestock. Sightings peaked in the 1920s, but have since faded, so perhaps Alkali Lake, now called Walgren Lake, is free from its fearsome, smelly unicorn beast.
You can see an artist's rendering of the Alkali Lake Monster and a few others at Atlas Obscura.