That depends entirely on what terms the oral folk storytellers used! I'd use "Nix"/"Näcken" whenever that's what the locals used, and a more generic "river spirit" if there are multiple names for similar entities.
To use a German example, I use the term "night hags" when talking about sleep paralysis demons. The German texts use all sorts of different regional names, such as "Trude/Drude", "Mahr/Nachtmahr", "Walrider", "Incubus", "Alp", and so forth, but these all fall into the same overall behavioral patterns, so "night hags" is okay as an overall group name.
And I see "dragon" as a group name as well. "Lindwurm", "Drak", and so forth might be more specialized names for the German "Drache", but these terms, too, mean different things to different people - and often the very same creature is called a "Drache" in a different story anyway.
As far as I can tell (though I don't know as much about British folklore), the " #dragon "/" #wyvern " distinction in the UK arose from heraldry, _not_ folklore. Heraldry experts needed clear definitions so that they could describe or draw heraldic symbols - but are those same definitions actually used within folk tales? Did the oral folk storytellers really distinguish between four-limbed wyverns and six-limbed dragons?
If they were _anything_ like their German counterparts, they would have used both names and appearances as narrative tropes that could be used or discarded as needed. And when it comes to #folklore , I will take _their_ word above the word of non-folk storytellers.
I hope I am making sense here.
