|I kind of want to see it, but it'd probably be garbage.|
Attacks: 2 Claws (1d6 each)
Saving Throw: 10
Special: Underwater, Regenerate (Slow)
The Gill-man is an amphibian-humanoid that is known to inhabit out of the corner lagoons in far away reaches. He (typically it's a he, though scholars, men of science, and sages are unsure how to determine the gender from a mere visual examination) is known to stalk fishing boats and the occasional research expedition. Attacking crewmen or kidnapping fair women (perhaps his one weakness). Here is how he could appear in various games / campaign settings.
Weird Adventures - Rumors of artifacts and dangerous denizens have reached the CityU (City of Empire University). To the men and women of paleontology this creature represents the first evolutionary step between the creatures that crawled out of the sea and modern man (if you believe in all of that science mumbo-jumbo as critics like to say). The party is a search-and-capture team sent to darkest Asciana to recover what might be found.
Scarabae - A lot of things get dumped into the streams and rivers that flow through the great city. A lot of sewers collect not just refuse, but magical runoff from alchemical works or factories. The Gill-man are perhaps Lizardmen or some freak creation created by the chemical spill. Now it haunts the waterways, snatching up people who get too close. Perhaps a sorcerous patron wouldn't mind for their personal zoo.
Borderlands - The Gill-man is a legendary boogieman of The Great Salt Marsh, not just by Humans either. Grippli have legends about this lone creature stalking Marsh and pulling down anything it can get its hands on. The Gill-man is especially viewed as a danger and menace to the Saurians (the Lizardmen of the Borderlands) who is said to be able to decimate entire spawning grounds. If there is truly a Gill-man is either a misunderstood creature or a muck dwelling fiend.