It’s hard to stand still when a 1,000-pound horse is galloping straight toward you —snorting, pounding her hoofs, white mane flying. “I should get out of the way,” I remember thinking. But which way? If I stepped right and the horse veered in that direction, I might get trampled. Same thing if I stepped left. I barely had time to think or breathe. I looked at the white mare as she came closer, our...