I have an undying love for the fey folklore, fairies are my jams and I love the fae myths, that's why a lot of my work is inspired on it. This time, I want to tackle on my "changelings".
Conventional fae folklore often refers to changelings as the creature left behind when evil fae steal a child from its crib. Here, the term “Changeling” refers to the stolen child itself. Changelings were humans once. Mere mortals stolen from this world and taken to another. In a land known as Faerie, they were twisted and changed. Turned into fae themselves, they exist in two major states. Those in service to their abductors (Loyal) and those who have escaped the madness of Faerie (Lost). They are often called the lost ones, or the stolen children; because that’s what they are.
So, to reiterate, Changelings are Fairies, kind of. More specifically, they're fey, and every last one of them was once a human, born on Earth, probably raised on Earth, and stolen away by creatures with god complexes and an alien-like monsters appearance to be transformed into slaves. Their souls, and indeed their personalities were shredded by thorns, and they served their usually terrible durance in Faerie in a mindless stupor, the more time they spend on Faerie, the more they become part of it, they lose their identity and humanity to the beautiful yet dangerous and cruel things that surround them on that world, and they begin to lose their memories, the ones from their real life. But on the rarest occasion, a slave catches a whiff of memory that overpowers their desire to stay in Faerie, and they work to free themselves by outfighting, outsmarting, outcharming or outbargaining their freedom from their Keepers. They earn their sweet liberation, and can begin the healing process to mend their broken bodies and broken minds. But even so, they have to deal with all sorts of problems. Odds are that either a long time has passed in the real world since he's been abducted, leaving him anachronistically stranded, or a short time, leaving him as an adult while his family expects him to be a child. Either one, none of them is good.
Changelings are neither human or faeries, they don’t belong anywhere, and that’s the tragedy of their existence. A changeling knows that they not longer are the same as their father, the same as their mother, their friends, the people they grew up with, a changeling is walking between two different worlds, and is not part of either.
Life as a Changeling really sucks. You get to enjoy PTSD and wonderful thoughts such as suicide and giving in to madness on a daily basis(delusions, phobias, depression and hallucinations are super fun guys don’t worry), on top of worrying when the true faeries will show up to take you back to their world, or whether your best friend is a hunter manipulating you into the hands of one the True Fae, or having to worry about multiple organizations of humans trying to capture you and use you as their sweet guinea pig. Luckily, Changelings aren't stupid, and they understand the value of teamwork and contact with other people, that's why they have this sort of groups that they call "Courts", these groups provides protection against the Fae and a much needed care and understanding between changelings because "These people went through the same as me, I'm not alone", which is very much appreciated, since Changelings really want to have a place to call home.
They can have many forms in the end, based normally around what their purpose was as slave back in the Fae Domain, but is normally based around various "subraces" per say, the beasts, who have gotten animal like traits, the darklings, those who have become dark itself, elementals, who have become the elements of the nature , the fairest, beautiful fairys closely resembling the Fae, the ogres/trolls, changelings full of brute force and the Tools who are inventors and innovators yet the weakest of them.
Changelings search for one and only one thing, is to reclaim their old or gain a new place and people to call home. The path to this, however, is difficult and full of thorns and is something that may be never completely reached, since the True Fae are far more powerful creatures, more than them, changelings, and more than even weaker mortals, they can only hope to be left alone and not to be found.