my own go on the mermaid, part mer-folk and human.
they're kinda from a short story I may do, where dark magic users are trying to catch mer-folk for potions, and to get away the mer-folk hide on land because the dark magic was hurting the sea as well.
the Mer-folk used their own magic to appear as humans, and started a town. after years of polluted waters from the magic real humans started to come to their sea-side town because how great the fishing was around there, from there it just grow. to keep from being found out, the mer-folk stopped telling even their kids about where they're really from, so they wouldn't blab it to anyone, because with their magic they also appear human. once the magic in the sea weakened most mer-folk returned to the sea, but there was some that stayed, with human relationships. over years the humans and mer-folk mixed, they still have their magic and true nature, so if one were to fall into the sea it may take over. turning the villagers into mer-people like this girl.
the forms they take will very, from close to mer-folk, to just patches of scales, but if they can take the change they can all breath in the water.