Okay, one of the nifty things about mermaids is how many different versions there are in myths across the world; from things that are more like simian sharks to haunters of sacred wells.
This is also, of course, one of the most frustrating things about mermaids if you’re trying to put them in a well-constructed fantastic world, instead of local myths and legends. Do you have as many versions of mermaids as there are in folklore? Are they different in every port, like folklore? Are they like humans, having culture and cities? And how does that sort with the realities of intelligent species as we know them in this world – where you need a certain minimum number of people to create a civilization in the first place?
There is, however, a potential answer to this problem, if you poke some of Celtic folklore. Apparently at least some Scottish and Irish mermaids can take their tails off – like a dress! – to walk around on land like mostly-normal humans.
So… what if the differences between all the kinds of mermaids are like differences between various human cultures? Basically the same species, a little ethnic variation here and there… but the various tails, fins, claws or lack of same, etc., might all be different dresses. Culturally different tails you can take on and off, that might also provide other features.
Siren that drowns unwary sailors? One kind of dress. Koi sparkly mermaid that swims upriver to the water markets? Another kind of dress. Shark-toothed ship-wreckers that raid like water-breathing pirates? Yet another kind of dress – like getting kitted up as a Mongol raider instead of an elegant court lady.
If you did this, you could have your myriad mythical mermaids and still keep them as a more plausible fantastic species. Culture can create swarms of differences; the examples are all around here on Earth. And then you also get an interesting question: if the types of tails are cultural, who makes them?
Bonus? Your mermaids can swap their roles. The elegant court lady can doff her shimmering koi scales and pull on the swift reflexes and deadly striking speed of a swordfish, or tuna. Or turn dark assassin with the venomous spines and claws of a lionfish… or worse, a scorpionfish. If she’s in a mood.
Heaven help your heroes if she has a good friend who’s a sea-snake medusa!