The typical depiction of a mermaid in D&D is like what you see in the Disney movie - upper body people, lower body big fish. They can swim really fast if and when your players ever go in the water, and flop around on the land waiting for someone to harvest them for XP.
So we'll keep the half-man / half-fish idea… but not make it so neatly divided. They can walk about on land, but they have webbed fingers and toes, gills, green scaly skin and so on. Not quite as inhuman as a Lovecraftian Deep One but a bit more inhuman than the Sub-Mariner.
The original D&D monster has an armour class equivalent to Leather, or Leather + Shield. I like this idea - that it's scaly skin is tough. Taking that concept further we'll also make them strong and clawed. Maybe a bit like the Creature from the Black Lagoon, or Abe Sapien from Hellboy.
So far it's still a fairly standard humanoid with a scaly green coat of paint though. That doesn't make it weird or creepy enough. Fortunately I saw this for the first time last night - and it's where we're going with the Mermaid Makeover:
So our "improved" Mermaid is also a Merman, and has the "takes people prisoner" quality of Dryads, Nixies, etc. Except a lot creepier.
