Human
I wouldn't say it was unwilling but I was led and used as an older boys cock sleeve for awhile. He wasn't unkind and was gentle so I was never hurt and don't really have any ill will to the experience. I do wonder if its why I'm as okay with kink as I am, but I feel like may date back even further.
Dog later on loved to lick ass, as I accidentally found out leaning over the bath. Later I was with a human lady for awhile and thankfully that is over.
No present lovers, but some of the stories here give me hope that maybe I can find a women willing to share a dog cock together happily ever after. Its a scary thought though because that is a hell of a thing to confess to someone. But I really have to wonder how common these feelings may be; people joke about it, furry art is pretty big, and there are even references to it in pop culture if you look enough. Maybe there are more curious people out there than I think.