So, the thing is, actively identifying as something is often one of the last steps in identity formation. I didn't start out identifying as a gnoll. It was a gradual process, of reflecting on my feelings, recognizing that I didn't really want to look human or live among humans, and eventually realizing that this desire too look and live as a nonhuman was actually a major part of my self-perception/identity.
I didn't wake up one day and think "Oh gosh, I'm a gnoll stuck as a human, whatever shall I do!" It was a slow awakening, of trying to understand my internal experience and figuring out how I wanted to express it, and what language could express it.
It was years before I went from "I wish I had fangs" to "I feel like I was meant to be a gnoll, like I'd be happier that way" before finally landing on "I'm a gnoll."
Of course, sometimes you do just wake up one day like "oop, guess I'm a bison" (happens to the best of us) but often the identification is the result of a long process of self-reflection.