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. Some stories apparently describe them deriving their ability to breathe water from magical caps, which they can loan as gifts or which can be used against them if you need to.
Dungeons & Dragons
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 the Monster Girl Encyclopedia, merrows are a specific breed of the mermaid family of monstergirls, who are mostly distinguished by the fact their ability to breathe water is bound up in their enchanted caps and by being straightforward sluts, in comparison to the more "romance first, sex second" approach of the standard mermaid.
|Dungeons & Dragons 1st Edition Races|
|Basic Set:||Dwarf - Elf - Hobbit - Human|
|Creature Crucible 1:||Brownie - Centaur - Dryad - Faun - Hsiao |
Leprechaun - Pixie - Pooka - Redcap - Sidhe
Sprite - Treant - Wood Imp - Wooddrake
|Creature Crucible 2:||Faenare - Gnome - Gremlin - Harpy |
Nagpa - Pegataur - Sphinx - Tabi
|Creature Crucible 3:||Kna - Kopru - Merrow - Nixie |
Sea Giant - Shark-kin - Triton
|Dragon Magazine:||Cayma - Gatorman - Lupin - N'djatwa |
Phanaton - Rakasta - Shazak - Wallara
|Hollow World:||Beastman - Brute-Man - Hutaakan |
Krugel Orc - Kubitt - Malpheggi Lizard Man
|Known World:||Bugbear - Goblin - Gnoll |
Hobgoblin - Kobold - Ogre - Troll