alterhuman is an umbrella term that covers various categories of people who feel they are, in some way, not your typical human being.
otherkin and therians, people who believe themselves to be in some way animal or mythological creatures, are an example of alterhuman identities. the term also encompasses plurality (also called multiplicity), people who believe themselves to be many people in one body.
as with any umbrella term, some of the people described by it don't use it for themselves for various reasons, which is fine. i find it quite useful as a shorthand, myself.