This isn't an introduction to alterhumanity; for that, see the sidebar.
My number one 'source' of being alterhuman is my clinical lycanthropy.
The link above is an introduction to clinical lycanthropy, but basically it's a somatic (physical) delusion (deeply held, yet false, belief) that the person is an animal. This is not the same as therianthropy or otherkinity.
I've written about my experiences with clinical lycanthropy on my mental health tumblr. My clinical lycanthropy presents as being a werewolf and a black low- to mid-content German Shepherd wolfdog, and when Cotard's is added I become a dead were/wolfdog.
I'm also transspecies - as much as one can be trans-species when one is already that species... (it just works).
My other kinds of alterhumanity are much more traditional. I'm cat'hearted (domestic cat, though questioning if my links to all felines are just paratypes or if I'm actually cladohearted). I'm probably also raptor'hearted (birds of prey, not velociraptor) (as a clade). I'm also coyote'hearted, as a result of my clinical lycanthropy.
I'm Indigenous, and my totems are dingo, wedge-tailed eagle, and crow (forest raven, specifically, but we call them crows anyway). I also have a connection to snakes through what I think are my family member(s) totem.
Finally, I dabble in dæmonism. My dæmon's Pullman form is a wolfdog, analytical form is a thylacine. Personally my dæmonism is more of a fun thought exercise than an actual entity, though I acknowledge and respect that for other people it's different. The image used in the header of this blog is of a thylacine, and (one of) the most common forms my dæmon takes.