TCFTBL (because it takes forever to type it out each time) is a 1954 Univeral horror film which is actually based on a Mexican folk tale. It was originally produced in 3-D, one of the first Universal pictures to be filmed that way!
In the movie, a geologist finds a fossil linking mankind to the sea, a fossilized hand with webbed fingers. He convinces his pal to mount up an expedition to the Amazon to find more of the skeleton. When the scientists (and one of the Dr's girlfriend,) arrive they find the rest of the team is dead! The movie then continues in typical horror fashion and the 'Creature' (also known as 'the Gill-man') goes for the girl and is eventually left for dead.
Gill-man was so popular that two sequels were produced; Revenge of the Creature and The Creature Walks Among Us. In fact he is still quite popular to this day. So popular that a TCFTBL musical was produced in 2009 and a new movie is in pre-production!
Here's a fun fact - "When Jenny Clack of the University of Cambridge discovered a fossil amphibian in what was once a fetid swamp, she named it Eucritta melanolimnetes, which is Greek for 'the creature from the black lagoon.'"
Personally, I always felt kind of bad for the creature. He wanted the pretty girl and was just misunderstood. I think that when he first walked into that camp he had no intention of killing the scientists, they attacked him and so the reign of terror that ensued was their fault.
Watch the trailer: