When a little foal with a white upside-down exclamation point on her forehead was born one morning in Kentucky, the heart of America's horse racing region, problems mounted quickly. Rejected by her mother, the filly would need to be accepted and nursed by another mare. As she grew, the tall, knock-kneed girl remained skinny and scruffy, with paltry muscles. Considered an "ugly duckling," she was unsuitable as a champion racehorse, her owner proclaimed,...