I can’t imagine that there would be a way for a person to be neurologically proven to be an otherkin – unlike being trans, which has a neurological basis that can be seen in scientific testing and backed up in psychological testing.
While I acknowledge that there are people who legitimately believe themselves to be animals or whatnot, and while I feel people have the freedom to self-identify however they want as long as it’s not at the expense of others, I don’t think otherkin-ness is a real condition in the way that being trans is.
I will say that I find otherkin folks less objectionable than transethnic folks, because (as far as I’m aware) otherkin people don’t tend to trivialize the oppressions of other groups in the same way that, say, a white guy who “identifies as black” is doing. But I know some trans people who feel that otherkin culture trivializes the very real condition of being transgender/transsexual, which I think is a valid point.