Kind of a double whammy. There was a dog that lived in the house right next to where I had to wait for the school bus in the morning when my family couldn't drive me. There was a missing fence panel. He'd stick his head out to get scratches behind his ears and he always got really wound up by the attention. Finally noticed one day that he wasn't just excited, he was EXCITED and it made me curious. But, I didn't really think much beyond "Of course dogs have dicks, too. How else are they going to reproduce?"
A good while later, I found a picture stuck in some grass on the walk home from school one day. Seemed like just a regular "centerfold" style pic. I stuffed it in my backpack to take a look at when I got home. When I got to my room, I unfolded it and sure enough, it was porn. The woman was very attractive but, I had already seen more than my share of Hustler and Playboy magazines by that point so nothing stood out; until I noticed that the Great Dane in the pic wasn't just posing with her.
He had his front paws on her shoulders, his cock was hard, and she had her hand wrapped around the base. It was huge and held my gaze for a really long time while I processed what I was seeing. I was turned off by it at first glance but, something about the image kept drawing me back. I spent weeks going back and forth between being disgusted by myself for being curious and aroused by what I saw. Finally, I searched the internet to find out more, discovered a lot of other content that seemed to normalize it (in my head at least), accepted that I liked animals more than as just pets, and the rest is history.