But neither the mermaid nor centaur have human legs. If they all of a sudden get human legs, that means they can also get a fish or horse head. So there would be 9 combinations.

Assuming that the head/legs can be either type, the seahorse can be a truly horrific 4 limbed thing with a fish head Lapras or an almost standard seahorse

This in comic form

https://www.instagram.com/p/CCOYYK2JJ32/?img_index=1