Some further references:

"White matter microstructure in female to male transsexuals before cross-sex hormonal treatment. A diffusion tensor imaging study" (Free.)

"Our results show that the white matter microstructure pattern in untreated FtM transsexuals is closer to the pattern of subjects who share their gender identity (males) than those who share their biological sex (females). Our results provide evidence for an inherent difference in the brain structure of FtM transsexuals."

"The microstructure of white matter in male to female transsexuals before cross-sex hormonal treatment. A DTI study." (Free.) This is by the same authors as the previous paper.

"Our results show that the white matter microstructure pattern in untreated MtF transsexuals falls halfway between the pattern of male and female controls. The nature of these differences suggests that some fasciculi do not complete the masculinization process in MtF transsexuals during brain development."

This web page speculates (based on no evidence) that the causal relation is the opposite: that such differences result from transsexuals' preoccupation with being the opposite gender. Cf. the known differences in taxi drivers brains that may result from their years of navigation experience. The speculation leaves unexplained the origin of that preoccupation, and appears motivated by the author's theory (which is what that web site is for) that sexual orientation is plastic.

Be all that as it may, there is a clear difference here between transsexuality and otherkin. The hypothesis in the last of these links, of voluntary (if unintended) self-modification, can apply to both. However, the involuntary developmental process suggested by the first two can only apply to transsexuality, since there is no developmental pathway for being an elf.