"The Horror at Martin's Beach" is a short story by H. P. Lovecraft and Sonia H. Greene (1883-1972). Also known as "The Invisible Monster", it was written in June 1922 and first published in November 1923 in Weird Tales (Vol. 2, No. 4, 75-76, 83).
Sailors kill a 50 foot creature at sea after a lengthy battle. The creature bears strange anatomical irregularities such as rudimentary forelegs and six-toed feet in place of pectoral fins and single large eye. 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 good samaritans and the captain are hypnotized and pulled into the water by the creature's mother to the horror of an onlooking crowd.
References in other Media
Peter Benchley's 1991 book Beast and it's made for TV adaptation contain a similar story of sailors killing a young gigantic sea creature and being tormented by its revenge driven mother.
The story has been included in two collections of Lovecraft stories: