Horseplayers at Pimlico were thoroughly perplexed as they tried to handicap one of the entrants in Saturday's second race.

Troop C, a 9-year-old gelding, had not run in a flat race since 1973, and had not completed at all since a steeplechase at Saratoga in 1976. Now, after a three-year layoff, he mysteriously was making a return to competition in a cheap claiming race. Perhaps supecting an audacious betting coup was in the works, the crowd sent him to the post at surprisingly low odds of 15 to 1.

If the public at large did not know quite what to make of Troop C, neither did his trainer, Bill Gordon.

Gordon is a real estate man who lives on a 100-acre farm in Potomac. His 23-year-old daughter, Lisa, operates the farm, and was responsible for purchasing Troop C last year. She did it quietly.

She always kept it a secret," Gordon said. "She kept him hidden. He was an old bowed horse, thin as a rail, and I think she paid more money for him than I would have approved of . . . I came across an old canceled check for $800 made out to a man in Philadelphia, and I think that's what the horse cost."

Though physically unimpressive, Troop C turned out to be an admirable thoroughbred in other respects.

"He's a smart old horse," Gordon said. "In his stall he's a gentleman, and when he'd go out to gallop he was all business. When we'd take him to the little training track on our farm, he loved to run."

One of Lisa's friends aspired to ride the gelding in a point-to-point race this spring, and so Troop C spent the winter training, even in the snow. He was so fit for the race in Potomac that even after being left at the post, 20 lengths behind, he caught the pack. Soon thereafter he won a Howard Country point-to-point race, emboldening Gordon to think that Troop C might have a future in racetrack competition.

That was a rather optimistic note, for Troop C's past was not exactly glorious-his only victory had come in a maiden race at Delaware Park in 1973-and horses rarely hit their peak at the age of 9.

But horses have a way of stimulating optimism in the people around them and Gordon began to think that Troop C might fare creditably in a low-priced claiming race at Pimplico. So he sent the horse to Baltimore for a workout to familiarize him with the track. Jockey Javier Canessa drilled him six furlongs and told Gordon the horse was ready to run well.

On Saturday, Gordon saddled Troop C for a $4,000 claiming race. Then he watched with delight as the horse broke surprisingly well and was running second as the field hit the first turn. For a moment, suspicious horseplayers in the crowd must have thought that, indeed, a betting coup was in progress.

But Troop C's moment of glory was fleeting. By the time he had run a half mile he was not out of contention and after three quarters he was last. An at the finish line was an ignominous last, 20 lengths behind the next-to-last horse.

That should mark the end of Troop C's racetrack career.But while other slow 9-year-old horses might be destined for the glue factory, he can look forward to a happy retirement, running in occassional hunt races and munching grass on a farm in Potomac.

For a horse who has managed to win only one minor flat race in seven years, life has treated Troop C well.