Their older designs had them more fish-like and they also had a different face and appear to have a collar or frill on them.
Merkid is one of the only Pokemon in Solar Light to be in a branched evolution line, with Baawool and Guppish being the other 2. Unlike Guppish and Baawool, Merkid can be captured in the wild and is a Gender based evolution, similar to the canonical Pokemon, Burmy.
It's name is similar to the name of a Fakemon from Pokemon Uranium.
It is based off a Mermaid. The ears might hint that they they also based on elves. The Shell on their heads forming a crown shaped hair clip is a reference to princesses and princes.