I've always found animals very attractive and felt more empathy towards them than humans. My empathy didn't stop at mammals either, as a kid I was very fascinated by insects, and for a while I couldn't believe they may not have consciousness the way we do. It just seemed like any living animal is a person, to me. It was difficult to think otherwise.
I realized the sexual attraction part in my early teens, and the trigger for that was quite funny and innocent: I was reading The Black Obelisk by Erich Maria Remarque, where one (human) character was described as having buttocks like a "baroque horse", I pictured that in my mind and it struck me - "holy shit, horse rumps are soooo hot!", for a while I only thought of horse butts when I was fapping. Afterwards my imagination diverged into different kinds of experiences with other mammals. I never really had irl sexual encounters with non-human animals, but the attraction and fantasies never faded.