I was 12-ish first time I fingered and later licked my family's elderly female mutt. Around the same time I played with some small pets (gerbils, guinea pigs / cavies, etc.) and a male cat or two.
A couple of years later, we got an intact female shepherd. She came into heat shortly afterwards, but she was more interested in humping me that letting me hump her. I couldn't figure out how to get my penis into her, but I sucked her to orgasm many, many times. Each time she'd had enough of that, I would wank myself and she happily lapped up my cum. After she finished her heat, my parents had her spayed. She still liked me to suck her afterwards, though.
A year or so later, my neighbors got a medium-large male mutt. He loved hand jobs and I let him fuck my foreskin and fill it with his cum. After a bit I added my own cum and it couldn't hold any more. My foreskin popped off the end of his dick and the mixture of his and my cum went all over. Messy but fun. We did that a lot until the neighbors moved away. I never got around to sucking him, wish I had.
I didn't get my own dog until I was about 21 and had my own place, when I got a male Corgi. I got another one a couple of years later. I did suck those boys, probably thousands of times.
Curiously, I didn't lose my "bitch virginity" until I was about 30. A neighbor's Pit x Lab bitch showed up at my house in full, standing "do me now!" heat. She wanted a cock in her, didn't care whose, and she wasn't gonna take no for an answer! My neutered boy took sloppy seconds after me, tied her and everything. I'm pretty sure he'd never had a bitch before.