I've been identifying as a therian since I was 13. I always assumed I'd grow out of it, or that it was just another fixation that would fade. But it never did, and has only grown stronger and more integral to my sense of self as I've aged.
The label I use has changed and morphed over the years;
It was therian to start, and then I added otherkin onto that for my fictional and/or concept based identities.
The last year or so, I settled on saying, definitively, I'm nonhuman. Whether I perceive that as biologically the case, or not, depends on who's most prominent in my system (I'm diagnosed with DID.)
Mostly of my nonhumanity revolves around canines of varying types, again depending on who's fronting. But as a whole, "Canine" is an acceptable catchall. Then there is, still, the concepts which I identify with -- the void, tech/computers, angelic, robots, etc.
I will say, the reason I specify nonhuman as opposed to alterhumanity for my own personal verbage, is because I'm not human, in any shape or form, left right or to the side a little, alternate or not. -- to be clear, this is how I experience and view myself in relation to those labels, not something I apply to others.
Anyways... It's a very important part of who I am.
And it's good to be able to talk about it, so thank you!