The horse tail is the action a woman makes when an unwanted appendage (finger or penis) is about to enter her rectum. The horse tail comes out exclusively during doggy style intercorse. While in position, she will wildly swing back one arm to swat the unwanted appendage from entering. It resembles a horse's tail swatting flies of its ass.
I tried to take this chicks ass last night, but her horse tail came out and thwarted my attempts!