I lost my virginity to a ewe when I was in my late teens. After reading an article in a "lad's mag" like FHM or Nuts or something about the parts of rural Colombia where young guys routinely have sex with donkeys, I had a bit of an awakening and started looking online for anything I could find, which wasn't much back then, especially not on 28.8kbps dial-up. I was home alone one day, looking at zoo porn, when I suddenly realized "I could actually do this instead of just looking at it."
We kept a few sheep at the time, which had been hand-reared and were very tame, so I took a bucket of feed, went into the field, put the bucket down in front of the friendliest of them and tried to get in position behind her to get inside. Unfortunately, in my extremely nervous and excited state I hadn't really planned ahead properly, so I had my trousers dangling off one ankle and one foot in a thistle, and she was so short that I had to kind of half-crouch, which was hell on my thighs, but after a bit of awkward fumbling, I slid in. I honestly don't remember much about it, because I was so high on adrenaline at what I was doing, but I don't think I lasted very long at all.
After I came, the reality of what I'd done hit me and I felt a fair bit of shame and went and had a shower and tried to pretend it hadn't happened, but I ended up doing it again a couple more times over the next few months. Looking back on it, I wish I'd had the confidence and comfort with zoophilia that I have now, because I would have had a lot more fun a lot more often, but what are your teenage years for if not to look back on with wistful regret?
I haven't had penetrative sex with an animal since her, though I have sucked/jerked a few male dogs, which was fun for everyone involved.