The Horror at Martin's Beach by H.P. Lovecraft

Sailors kill a 50-foot creature at sea after a lengthy battle. The creature bears strange anatomical irregularities such as a single large eye and rudimentary forelegs and six-toed feet in place of pectoral fins. After inspection by marine biologists, it is revealed to be just a juvenile. The captain who captured the creature tours the coast and profits from the corpse of the deceased creature. As the captain attempts to finish his business at Martin's Beach, a group of swimmers are attacked. The captain and others attempt to rescue the victims but it is too late. The rescuers and the captain are hypnotized and pulled into the water by the creature's apparently vengeful mother, to the horror of an onlooking crowd.