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So, I never finished summarizing my kintype research here. Stuff has been going on and taking my time and energy from otherkin things. Basically I have two other regions I looked into the myths of and I'll put the both here. Two things I didn't get from my research in Celtic and British myths that I feel are central to my kinity: the phantom shifts and the purpose. In those other myths the changeling had a purpose, but it related to the fairy desire for the child not the actions of the changeling. And they never took on a form other than the child they impersonated.
I finally found some German myths that weren't Grimms fairy tales or Nordic legend. A few changeling stories that apparently dated to around the time of Martin Luther, since he is referenced in several as having identified at least one changeling. Some earlier stories about earth women (dwarves) changing our children give some examples of changelings living to adulthood, and actually growing to be adults instead of giant 20 y/o babies, where the family is promised goid fortune if they raise the child well. Not necessary, but it makes me feel better because my gut instinct is to dismiss myths that have changelings not surviving to adolescence as having nothing to do with me.
But better is the later legend of the killcrop. Malevolent elves, the devil, or Rye Mother (look up that last myth if you love creepy stuff) would switch human children with spirits that would bestow misfortune to their families. They wouldn't just be misbehaved, they would poison their hosts and livestock, break valuables, and mystically destroy crop fields by their presence alone. Hence the name killcrop. I've always felt that the entities that left me here had a malign purpose for me, but the partial intructions embedded in me are vague and ill formed beyond the notion of a basic requirement for destruction. And the purpose of this type of changeling is non specifically destructive. It works really well, though it is easy to match up two vague things. They are also describes as often growing like a normal human, so points there. However, they don't have any non human physical features, which brings me to my next region.
I found Slavic changeling myths by changing up my search terms to "do demons leave changelings". I know that's biased towards particular results, when part of my purpose in research was to determine if demonkin was a good fit, but I was getting nothing about demons being involved except for the occasional aside about "oh yeah later demons were described as doing this too" and I wanted specifics to compare to. The Slavic myths were some of the first results, and it brought me to sites that described both demonic and non demonic changelings so I'm happy. Apparently in Russia water spirits will replace your children, for reasons not too different from the "take care of our elderly" motive of the Celtic fairies.
The demonic versions are given the same purposes as the killcrop, usually. Sometimes however they are tricksters and tempters. They convince the family to raise them (and supply them with endless vodka) in exchange for riches and magical powers. Riches that of course turn into dirt when the changeling leaves. The expression of magical powers by these changelings was new, and I hadn't thought about it, but it fits with some other sensations I've had of abilities I "should" have. But the cincher is that many demonic changelings in Slavic myths grow features such as horns as they age that give them away. I.e. matching features to my phantom shifts (not the wings but, adding those is not a huge stretch).
I consider this enough to confirm for now my suspicions of my kintype being demonic. I haven't checked legends from non european cultures much, there aren't many that are analogous, but for completeness that's the next step.
So, verdict: demonic changeling. Neat. I'm too tired right now to decide how I feel about the confirmation and this post is long enough so end scene I guess.
So, this was delayed due to life reasons. Time to get back to it.
On my first library trip to find more details, and hopefully actual changeling myths instead of the summaries I had found online, I pretty much just got tales from England and Ireland. It's a public library not a university one so I guess I had unrealistic expectations as to the variety of its contents. But I did find some of what I was actually looking for: transcriptions of actual myths.
The best I found was a scholarly source with both written and oral traditions. Its contents were discouraging. None of the descriptions had the changelings possessing non-human forms, so nothing to match my shifts. Also none of them grew beyond early childhood, in the one story that had a family keeping a changeling for 20 years it was an (enormous) infant the whole time.
My other point of information for myself, the feeling that I've been placed in this body for a particular (unpleasant) purpose, matches nothing in these stories. Where a reason for changelings was speculated on it was fairly benign. Fairies being unable to have children, care from humans for the elderly, fairy love of beautiful children, and a specific desire for human breast milk all came up, but none of those click with the thoughts that bubble up that I consider my vague "instructions". In fact several of them involve older fae coming to the human world to replace children, which would include memories of their lives beforehand. I have no such kin memories and don't expect to develop any.
When it comes to physical descriptions, the makers of folklore and I are really working at cross purposes. They describe changelings as being ugly, ill behaved, learning to walk and speak late or not at all, and dying young. I can dismiss these because people are trying to explain the world around them, understand why the had a child with a disability or illness, not give explanatory descriptions to someone trying to figure out why they don't feel human. I don't feel like I can use these "mundane given supernatural explanations" stories for understanding my own changeling identity.
Which means the stories I've found up to this point only give me a strong pinging of similarity, and not much else (plus the intellectual curiosity but yeah)
I realized this is probably a better place than the forums for musing on and documenting my research into kin stuff. So that's what I'm going to do. This will take a while because in addition to what I've already posted I have another chunk from this past week and further thoughts on what I had before. So yeah.
Starting off in my research, I wanted to have a solid foundation for what I'm looking for, to minimise being influenced by whatever I saw (sort of how med students start diagnosing themselves with all the things). My phantom shifts are important... but creatures with wings and horns are wayyyy too common for that to be a helpful starting point. And going with the specifics, the wings are skeletal and batlike, got me a bunch of nothing.
I really feel the need to give the changeling aspect of my identity primacy. Whatever it is I turn out to be kin to, those feelings came first. I am not the original or intended owner of this body. I have a purpose in being placed here. These are the thoughts that come out of my subconscious in a voice not my own. Feelings I've had since early adolescence. I identified as a changeling for years before otherkin. Shifting came later.
So what I've been doing is looking into baby-swapping myths and trying to match that up with my shifts and other information. That's my starting point. End part the first, more later.