where does one determine where to draw the line?
Short answer: By what the experts say, and they say transgender people should be treated as the gender they identify as. Period.
Long answer: A lot of PhDs did a fuckload of research over a century plus and showed that, yeah, gender is super fucking complicated and doesn't map out to male/female based on your genitals at birth (let alone for the reason that, you know, maybe you might be born with a penis AND a vagina or ovaries AND testes or female chromosomes AND male genetalia, etc.), and if people get some simple gender affirmation, they live better and happier lives, and that applies to cisgender people, as well.
Easy answer: You can claim to be fucking anything you want. Who actually gives a shit? Let people be themselves if it don't hurt anyone. What's the problem with being a transgirl or a transboy? Why do we even have multiple bathrooms? That just seems to punish all sorts of people for no reason.
If you WANT to say transracial or transspecies or transnational is a thing, by all means do some research and prove it through studies and peer review. Until then, it is unlikely to be recognized the same way that transgender has because it has a lot of supporting evidence.
I think you misunderstood, but I'm not presupposing otherkin isn't a thing. I am saying it doesn't have the same type of intellectual backing as transgender experience does, so it isn't treated the same. I think that is unfortunate, even if there are studies done as well as expressed experiences, especially within indigenous peoples (and you could argue that is part of the reason fewer studies are done on it.)
I'm not really here to debate whether fish exist because I know fish exist and I can drive to most lakes and find fish in them and I can go to a few museums and see fish remains and I can go to pet stores and find fish for sale and I can go to a grocery store and find fish to eat. Doing that same thing with people and their personal experiences is much harder since it's more of a personal experience and not, you know, a visible phenomenon, and so it's going to be harder to convince people a personal experience is real if it's not their experience and especially if it's not a common one.