You began to feel more pain in yourself, as you felt something coming from behind your butt, and finding out that you have grown a tailbone that has become longer than you thought it would go. Your own butthole began to pucker in between your massive new breasts, and your extending tailbone, and you begin to see what happened next.
The tailbone you have grown behind yourself began to grow hair, as you found that this hair was the same color as your own, and could feel skin underneath the tail form, until you found that it was long enough to be a horses tail to match your horse shaped penis. You were so horny from growing the tail, that you forgot about the other changes that would form on you, and decided to just masturbate, as you continued you feel pain in yourself with how much you were altering.