AT the Common Ground Fair riding ring, people lean over the high wooden fence to watch a bizarre sight: a former racehorse makes tentative, jerky leaps over small jumps - a bit confused about where to put his feet. Such exercises are understandably strange to an animal that has pulled two-wheeled sulkies at the racetrack all of his life. But a career change is worth a try, since the alternative is literally "deadly" - the dog food factory. In keeping with the fair's recycling theme, Standardbred racehorses past their competitive "prime" are being displayed at the fair to show that with reeducation they are capable of new pursuits. "Why let a perfectly healthy horse go to a slaughter house for dog food?" says Pat Lamperta, sitting atop her six-year-old trotter. Ms. Lamperta's horse was three when she bought her. At the time, the racing trainer said the horse had "overextended herself," and wanted to "retire" her. But through the Adopt-a-Horse program, run by Diane and Jim Parsons of Palermo, Maine, the animal was saved. "I got her for $350 with papers," says Lamperta. With the new name of Dejavu, the horse now does dressage and cross country riding. "For every horse we save, there's probably 10 that get shipped for dog food," says Mrs. Parsons. She and her husband buy horses from the track for their "meat price and then find homes for them. Many people think Standardbreds are too high-strung for riding. At the racetrack high-energy grain and sometimes drugs are used to "pump up" the animals, Mrs. Parsons says. But once removed from that environment, they are found to have excellent dispositions and make good pets, jumpers, mounts for police, or pleasure carriage horses.