I was talking to a friend last night, and she mentioned one of her friends was a "furry". Exactly what a furry is, is hard to explain and any reader who doesn't know is best off searching "furry FAQ" on the internet. Anyway, I knew what a furry was and then she mentioned that a friend of that friend was an "elf". Slightly intrigued I went to the internet this morning and eventually found (amongst all the dead links) an "Otherkin FAQ", which I found moderately intriguing.
Basically it's the belief that an individual's "soul" at another time inhabited a different body on some far-flung planet or dimension. Slow realization comes to these people that they had at one time been something other than purely human. Otherkin archetypes are reflected in human mythology with "species" such as elves, dragons and other mythical creatures.
In terms of "scientific falsability" this belief is impossible to prove or disprove, but like most of psychology, ethics and morals it is no more valid or invalid that any other purely mental phenomenon or construction. This includes such memorable diagnoses including Gender Dysphoria. Whilst I don't follow the Otherkin beliefs, I (by dint of my own situation) feel sympathetic towards them.
I presume that a person who disclosed their belief that they were an Otherkin to a psychiatrist might be diagnosed with something like "Disassociative Identity Disorder" or "Socially functional with mild delusional schizophrenia" because of the perception of the psychiatrist that this person is something other than that approved by psychology and "normal society" (the fickle arbiter of acceptability and normality). However, psychology itself can be no more or less validated than the assertion that one has a soul, or that that soul came from another far away place.