Merrow
Merrows are a mythological monster from Ireland, and are essentially the Irish version of the common merfolk. About the most unique aspect is that they explicitly have male merrow, who are described as ugly drunkards and that's why the beautiful female merfolk prefer to try and woo handsome human men instead.
In Dungeons & Dragons, Merrows traditionally fall into that "take a surface monster and put it underwater" approach to design, ala the koalinth. In this case, the classic D&D merrow is a water-breathing ogre that terrorizes the seas, a depiction that Pathfinder has preserved.
In 5th edition D&D, however, merrows were changed into a deformed race of ugly, evil merfolk who were driven mad and corrupted by unwittingly becoming worshippers of the Demon Prince of the Sea, Dagon.