Sahuagin
Sahuagin are a race of evil aquatic humanoids from Dungeons & Dragons. Rapacious predators who consider it their manifest destiny to conquer the oceans, they are known as the sea-devils and as shark-men for their malevolent "shark-like" personality, violent hatred of weakness and infirmity, rampant cannibalism, and ability to commune with sharks, although in terms of looks they more closely resemble the Creature from the Black Lagoon with a tail. They were one of the "big races" in Advanced Dungeons & Dragons; like Orcs, they received a huge entry in the Monster Manual, and went on to have their own book in the Monstrous Arcana series, called "The Sea Devils". Unlike Orcs, they have a more Nazi-esque mentality, with everyone knowing their place and working for the greater good (well, greater evil), and so they have a racial alignment of Lawful Evil.
Notorious for their propensity to have mutations; four-armed sahuagin are extremely common and usually rise to the highest positions (those seven attacks are a killer!) and the infamous malenti are sahuagin who perfectly resemble sea elves
It's also noteworthy that the vast majority of of aquatic good or neutral races are limited to a small habitats like shores or rivers or the underdark, are isolationist, or in the case of Sea Elves, not only have malenti infiltrators to deal with, but are in comparison? Dumb. That's right, Sea Elves have intelligence 8 (less than humans or land elves!)- and Sahuagin have a 14, making them a species of geniuses. Combined with being numerous, organized, and having their own patron god who doesn't mess their society up with mandatory backstabbing, Sahuagin implicitly own the sea elves in most D&D worlds. Their rivals are largely tritons and merfolk.