LONDON -- The poised 15-year-old stepped into the arena Thursday as the crowd fell into a polite hush. For six minutes she pranced and pirouetted just as she had trained, and when it was all over her team pronounced the performance a smashing Olympic debut.The teenager in question is the horse Rafalca, the most famous American athlete on more than two legs at these Olympics.The horse's renown is due only partly to its prowess in the sport of dressage, best described as an intricate form of equine dancing.In the stands was Rafalca's co-owner Ann Romney, the wife of Republican presidential candidate Mitt Romney, whose connection to the world of big-time equestrianism has prompted interest in the sport unlike anything American riders have seen before.Ann Romney "was absolutely ecstatic" after the German-bred Rafalca placed 13th among 25 horses that appeared Thursday, said Kenneth Braddick, a family acquaintance who edits the website Dressage-news.com and attended the competition. Three more U.S. horses are set to go Friday, with the top seven national teams advancing to a final round Tuesday.In an election campaign that has focused on the economic woes of average Americans, the regal Rafalca has unwittingly provided plenty of fodder for Mitt Romney's Democratic critics. Dressage looks, and sounds, like a sport for the 1 percent.The origins of dressage -- whose name comes from an old French word meaning "to train" -- date to ancient Greece, but it was revived as an art form during the Renaissance in the courts of European kings.World-class competition horses are valued in the millions of dollars, and the cost of housing and caring for them can run to more than $50,000 annually. (The Romneys reportedly spent $77,000 a year for Rafalca's upkeep.) No surprise, then, that when Mitt Romney was asked on the eve of the Olympics whether he would attend Rafalca's performance with his wife, he sounded like any husband who was trying to get out of a night at the ballet."I have to tell you, this is Ann's sport," Romney told NBC News. "I'm not even sure which day the sport goes on. She will get the chance to see it. I will not be watching the event. I hope her horse does well."