Well mermaids traditionally have a Human-like upper body, if they were to be in my fictional universe then that would mean they would have been the result of a evolutionary split off from Humans to explain why they look like Humans on top. But the question is how did mermaids come to be? It certainly wouldn't be from thousands of Humans mating with Fish.
Obviously the traditional Anthro make sense as something special evolved them. A fish of course would just become an Anthro fish like this beauty,
www.furaffinity.net/view/16319…However hybrid races I've not thought about, story-wise my main raccoon character has a girlfriend who is a snake which would mean they are having an interspecies love relationship.