Many years ago I was with a girl I was seeing from time to time. I knew she was bi, and had a girlfriend that was a more regular visitor than me.The girlfriend, lets call her Marie, was too butch for my liking, but 1 night I was with my friend and accidentally met Marie. After a few drinks we went back to Marie's place, where I was intruduced to a lovely german sheppard pup. A few more drinks and the 3 of us ended up in bed together.
I wasn't really into Marie, but the temptation of the 3-some was too much, and eventually really got into it, while the pup played on the floor.
We were in a small "daisy chain", with me as centre-piece, and we were giving it holly. Marie was giving me head, my friend was giving her head and I was giving my friend head, and Marie was rapidly becoming the most beautiful dyke in Europe, when the most foul smell I ever had the misfortune to encounter filled the room - the bloody dog had suddenly developed the scutters and crapped all over the bedroom floor!
My fantasy of a 3-some, with 1 a genuine lesbian and the other bi-sexual had been realised and shattered, both at the same time.
Probably not the animal sex story you were looking for, but that animal is now well dead, and so is my fantasy, forever!!