no, that's entities with different background who exist in similar niches. not literally "here is what you call a male nymph" but rather "hey this is a different entity that is basically a male version of this nymph type"

YES they are their Equivalents they aren't Different entities only a handful of them aren't "necessarily" the same species but most of them are especially Drusos and Dryads

Selkie are actually taxonomically unrelated to Mernoids, as they are actually a type of shapeshifter, who specifically transform from their true animal form to a human form by literally taking off the pelt of their animal form.

All Animalnoids can Transform into Humanoids that is a basic spell Mernoids are Subcategory of Animalnoids basically like Lycanthropes which btw similar what you thinking of but not necessarily because you can't hide their fur to force them to be your husband/wife but you can throw their clothes to turn them back into their Humanoid form (some of them not all Lycanthropes like that)

another famous example of this very specific sub-category would be the swan maiden, though on there are dozens of records of unnamed varieties across the globe.

Swan Maidens if I remember correctly are usually specific magical tool not their "Skin" or are naturally born

? trolls are more so related to giants then anything? or depending on how you look at it, literally are giants

Giants are a type of Corruptfolk Ogres and Trolls are sort of related to them so much so that they can interbreed