How I knew what I was is a complicated question.... I grew up knowing that something was 'different' about me, but not really knowing what. I was lucky to be able to grow up spending my summers and winter breaks essentially in the wild. While some kids built forts, I built dens... I didn't realize that was what they were until later (I thought of them as 'forts'). Forts generally are not grass lined scrapes under large rocks, just large enough to fit comfortably in.
I could run just as fast on all fours as on two feet (and often did).
I often felt uncomfortable in my body... and still do. It's odd to look in the mirror and not really connect to what you see reflected back to you. I've frequently wanted the old horror movies to be real and to be able to change my body to reflect my real self. I realize this isn't possible and I've made peace with that. (and no, I'm not into beastality... unless my attraction to my mate equates to a sort of reverse bestiality)
I always knew that what I was doing was "wrong" somehow... and that I shouldn't let anyone know about it... unless we were "playing" animals... or later playing "monsters". As I got older I tried to stop, several times... but I always 'relapsed' into my comfortable animal ways. And always made sure that I was safely out of view of anyone who might discover my secret.
I even hid it from my mate at first... I didn't want her to reject me for being 'crazy'. (Ironically she was doing the same thing... but she was transgendered) We came out to each other around the same time and we have often talked about how parallel our experiences were growing up.
She is in transition which is wonderful... now she can live full time as she truly is. I have a significant measure of peace knowing I don't have to hide my nature from her and our home is my safe space. I do have to admit that I now worry a bit about how to be around my son. I don't want him to be like me... I want him to be "normal"