KUNA, Idaho -- After watching a too-small thoroughbred finish dead last for the sixth straight race, 13-year-old Sara Mittleider convinced her father to buy the washed-up racehorse for $300 to replace the pony she had been riding at their 20-acre Idaho farm.

Seven years later, horse and rider are among 12 finalists competing this week for a spot on the elite U.S. team going to the World Equestrian Games in Germany next month.

Mittleider is one of the country's rising stars in three-day eventing, an equine triathlon that combines the ballet-like discipline of dressage, the endurance of cross-country steeplechase and the technical athleticism of show jumping.

"This sport evolved out of the military and it's still very much a military test," said Torri Nohorniak, managing director of eventing for the Kentucky-based U.S. Equestrian Federation, which selected Mittleider and 11 other riders for the national selection trials under way this week in Allentown, New Jersey. Friday, judges will name six team members to compete at the World Equestrian Games Aug. 22 to Sept. 3 in Aachen, Germany, second only to the Olympics in equine sporting events.

Mittleider, now 20, rode to the top of the eventing world on an unlikely mount -- a short, small bay-colored gelding named El Primero that she calls Tony.

"I saw him in the paddock at the track and just loved his attitude," she said. "He was not fast, didn't even attempt to run, and was just ornery."

Her father Gary gave in to her pleas and bought Tony with the sole intent of reselling the horse as quickly as possible.

"Sara was really mad at me, because she wanted to keep him, but she wants to keep them all," he said. "I'd have sold him if anyone would have bought him, but he was too small."

But Tony was the perfect size for a 13-year-old girl who'd taken her first horse-jumping lesson at age four. A year after she began riding Tony, Mittleider discovered the little horse was fearless when it came to jumping, leaping the 4'6" tall fences that are used in competition.

"Dimensionally, it's about like jumping a horse over the bed of a one-ton pickup truck," says her mother, Brenda.

Sara and Tony began competing at higher competition levels, traveling around the country to events and serving notice on the equine sporting world.

"Sara first came onto the scene in 2004 when she took everyone by surprise at Fair Hill (Maryland), where she finished 13th," said Nahorniak. Last year, Mittleider finished 18th in the Rolex Kentucky, the top young rider in North America's premier equestrian sporting event.

But a few weeks later, Mittleider was faced with the painful prospect of euthanizing her horse. While lying in his pasture, Tony rolled over a rock that damaged the nerve in his right front leg, leaving him partially paralyzed and unable to put any weight on the leg.

"He had to put all his weight on his other front leg, which started to damage it to the point we might have to put him down," she said. "This time last year, I wasn't sure if my horse was going to live."

Tony managed to recuperate by lying down for hours at a time, finally able to put weight back on his right leg after nearly four weeks. Mittleider jumped him just once in training before embarking on a five-month circuit of eventing competitions in Alabama, Florida, Georgia and Kentucky, the latter where she finished 14th in the 2006 Rolex and received the award for best-conditioned horse.

"That's almost better than winning, having the best-conditioned horse," she said. "He's such a cat with springs in his legs that I don't have to slow down as much before each jump like the bigger horses do, so I'm able to keep a more steady pace and finish with a strong horse."

The family has turned down several six-figure offers from people wanting to buy Tony and Mittleider hopes to ride her "itty bitty" $300 horse in the 2008 Summer Olympics in Beijing.

"He wouldn't do this for any other rider," said Mittleider. "When we jump, it's almost scary how well we work together."

On the Net:

U.S. Equestrian Foundation: http://www.usef.org/

2006 World Equestrian Games: http://www.aachen2006.de/default--e.htm