When I was in college, I was definitely more tame than I am now. But that began to change around my second year where I met and began dating a girl, we'll call her J. J was definitely kinkier and more experienced and forever changed my life when she opened me up to so many wonderful kinks (I also learned some hard no's of mine).

Anyway, on to the confession. A few months into dating, we had gotten into pet play with tail plugs and ears, collars, and she was all in on the whole experience (acting like a sweet little pup). This evolved into us buying a k9 shaped dildo and using it on her. That led to me using it as my own cock at times.

After a while, we were both super into it and she confided that she was always curious about her dog's cock (was a larger breed). She said she's seen it a few times just from his own natural excitement. We decided to start with simply not kicking him out of the room when we played around, which naturally led to him sniffing her after we had sex and eventually licking her pussy. That continued and became more intentional (never forced, he always came up to her freely).

As time went on, it escalated to her rubbing the dogs cock and licking it to eventually giving it head while I fucked her with the dildo (or my own cock).

We did try, on a few occasions, to have him mount her, but he didn't seem interested and we didn't have any desire to train or force it. So for the remainder of our relationship (which ended for unrelated reasons) we would incorporate her dog whenever he was into it and around.