I'm a metamodelist, so frankly, if there is another theory that can make sense, I'll entertain it during specific circumstances.
So that means I sometimes entertain/believe in the spiritual models, which can include Reincarnation in a wide variety of forms. The most commonly used form in the west, that an objective and self-contained Soul moves from body to body. A more Vajrayanic perspective, that there is no objective Soul, or even if there is, that memories, abilities, drives, and personality don't have to incarnate with the Soul (if there is one), meaning people may not be a Dragon, but have the Karma (real Karma, not the West's Karma-light) of a Dragon, or just the memories of a Dragon. Particle reincarnation. Etc.
Psychologically Otherkin can just be the mind finding an appropriate symbol set to understand itself through. As I've told people, if there were some way to properly disprove my nature as a Rakshasa on a spiritual level, I would still call myself a Rakshasa, because other than Rakshasa, there is only one label that is more suiting for me, and that's Ges. It's not so much about DID (though I'll include that a possibility), but that in order to take all the disparate parts of a personality and make sense of them, the mind finds/creatures a mythological model that fits it the best and identifies as that. It's how our minds work. We constantly see characters in books, and movies, or people in real life, and identify with and compare ourselves too, Otherkin could be the same thing playing out.
It can also be imprinting and cryptomnesia, being exposed to dogs as a young child, and documentaries on wolves and canines, can imprint the behaviours and patterns of a canine (at least as understood by a human) into the mind, which can then later be explored and understood as part of the identity.
Cybermodels (though this also fits in Energy models), it could be that someone's mind operates on the same "wavelength" as a specific being (not just a Wolf, but Greg the wolf in Alaska) and as such there is a connection between their minds, as they are in resonance. So it's not so much that Kin are objectively what they think, but they're just resonating with a specific being by fluke, and taking that idea into their identity (and somewhere there is a really ineffectual demon who is resonating with a human). Meaning (I'll pick on Miniar as he is the only poster so far that I know what he is, without having to go back a page and look at his profile), that Miniar's consciousness has a specific note, and somewhere there is a Rakshasa that has the same consciousness note or one really close, and like two tuning forks, they are just resonating off of each other, two forks, one noise.
Biological theories, can range from "Elves were real, and I'm part elf" to notions of atavistic patterns left in our hindbrain, which if activated we'd identify with creatures we consider appropriate (even if they aren't).
Frankly, while I lean toward a Soul Model, I think it would be foolish to discount the others. I mean, just because some souls came from elsewhere, doesn't mean Jimmy didn't watch too much Jurassic Park, and now is some reptilian/dinosaur/dragon kin, or that Maria, just by virtue of being artistic and regal identifies with Elves (even if it is purely symbolic and she's not aware of it), or that Liz isn't thinking he is an Angel, just because he shares a similar energetic "note" to an Angel, and they are picking up each other. Etc.
I can't remember the exact quote, so I'll just create my own. "If you can only understand the phenomena from one perspective, you don't understand the phenomena."
I think there is great value not just in having a theory about things like Otherkin, but being able to intellectually entertain other theories, just to see what impact that has on the object in question, or other parts of your reality.
(Yes, a post from Ges...depending on how swamped my life gets, there may or may not be more coming up again...hopefully earlier in the evening when I'm more coherent and trust my grammar better)
When we first begin all things simply are.
As we grow all things are external.
As we learn all things are internal.
As we understand all things are not.