I’m pretty sure it’s a common thing in folk lore and mythology. I think I read something about faeries having human mates at one point so I think it might be a common thing. This is not exactly my area of expertise but maybe @sarahviehmann knows more…?
At the same time, I think Rhapsodic is definitely heavily inspired by ACOTAR. It reads like it’s fanfic that somehow managed to make it to print, pretty much and the scene where they find out they’re mates is really similar to Feysand’s. In a sense, Rhapsodic is like a knockoff ACOTAR so even if the whole ‘mate’ business is a common occurrence, I don’t think it’s a coincidence that the book is so similar to ACOTAR.
Hmm, actually, no, the “mates” thing is nothing I’ve seen in any lore about the Fair Folk. They’ll take human lovers occasionally, usually royalty (especially in Celtic myth), but there’s really nothing about mates.
To the best of my knowledge, the use of “mates” in paranormal/fantasy books began fairly recently with werewolf stories. I’m thinking of Blood and Chocolate by Annette Curtis Klause specifically (it was published in 1997). I’m not sure if it was present in werewolf fiction before that, but it seems to be a pretty common trope in werewolf stories (Twilight used something similar with the Quileute wolves imprinting on specific people). I think it’s so heavily used in werewolf fiction because werewolves are animalistic in part, and the idea of “mating” is very sexual and is connected to that animalistic side. It could honestly be really interesting researching why the term “mates” is used in paranormal/fantasy literature to either excuse or set-up hypersexual behavior in intensely monogamous relationships.
The idea of mates being attached to Fae seems even more recent and comes with a renewed interest in mythology about the Fair Folk … though these new interpretations are not necessarily grounded in any established myth (frankly, SJM’s fae have next to nothing in common with actual Celtic Fair Folk or similar creatures). It seems to be more of an attempt to make an opportunity for exploration of more “animalistic” sexuality and destiny without having to deal with the whole wolf thing, too. All I can say is, at least we’re not mistaking the Fair Folk for pixies anymore. *shrug*