Well kiddo, a "Montauk Monster" is a creature that was found on a beach in Montauk, New York a few years ago. It was really weird-looking and people weren't sure exactly what it was.
Some people think it might have been a raccoon that got washed up on the shore and looked different because it was in the water for a while. Others think it might have been a turtle or some kind of mutant monster that nobody has seen before.
But the truth is, we don't really know for sure what the Montauk Monster was. Scientists and experts studied it and couldn't find any clear answer. So it's still a mystery.