As I mentioned in my intro, for me it is the result of at some point letting our (female) border collie start licking my penis when I was in my early teens/late tweens, which pretty quickly lead to my first orgasm. Is initially took place while I was home during school holidays while my parents are at work.
Because my sister (one year younger) also stayed home with me, combined with the fact that once I started it felt too amazing to not be doing it all the time, she never really caught me and quickly became a fan herself.
A border collie was extremely well trained, my Dad I trained several dogs when she was younger to competition level, and although a border collie was never entered into competition, we both knew how to train her and she was a very easy to train. So essentially after using treats just a few times to each her to lick a certain part of the body, she would understand and do it on command until instructed to stop from then on.
I'm 38 now, so this started in the mid 90s. Probably summer 95-96 (dec-Jan). While did get internet at sometime in '96 and broadband (cable) in '99, what porn there was/I knew how to find, at least in the dial-up days, was pretty team and mainstream. It wasn't until maybe the last 3 or 4 years that I started to get into watching zoo porn. I enjoy watching both women with their dogs as well as guys been linked to orgasm by theirs (this gebre is a lot less common).
I love how pretty much every video shows a loving relationship between person abd their pet, with both appearing to be enjoying themselves.
Happy to answer any questions anyone might have about my early experiences...