I have been collaborating with a friend on a project, which needed mermaid sprites (both above the water's surface, and underwater). He does not have an account on rpgmakerweb.com, so I am sharing the sprite base he created with permission. Other sprite bases may come in the future!
The fins are from Pandamaru's mermaid sprite base, here:
Terms of Service: This is an edit of MV's sprite base, using parts of Pandamaru's mermaid sprite base; you need a legal copy of RPG maker MV to use these resources. Non-commercial use is free, with proper credit given, but please contact Pandamaru for any commercial use (see Pandamaru's Terms of Service). Can be edited however you want (but please given proper credit for the original base).
Credits: Jeedai and Pandamaru (no need to credit me, but should you want to, use nofrodelius).