This has not been an inspiring year in thoroughbred racing. The horses who were supposed to be the stars of the 1981 season -- Genuine Risk, Jaklin Klugman, Lord Avie -- all have flopped. Pleasant Colony looked likely to win the horse of the year title virtually by default, but he demonstrated his limitations Saturday when he finished fourth in the Marlboro Cup.

But now a genuine star may be appearing on the horizon. Television viewers got their first glimpse of Before Dawn when she demolished some of the country's best 2-year-old fillies in the stake that preceded the Marlboro Cup. Having scored five straight victories over members of her own sex by a combined total of 29 lengths, she will attempt a rare feat next month, running against colts in the Champagne Stakes. No filly has won that race in 55 years. If she is successful, she will probably be the future-book favorite to win the 1982 Kentucky Derby. No filly has ever occupied that role.

Trainer John Veitch could not have imagined any of this just a few months ago. Before Dawn was overshadowed by a score of better-bred, better-looking 2-year-olds that Veitch trains for Calumet Farm.

"She didn't look the part, she didn't act the part," Veitch said. "Only once she showed a little flair in the morning when she outworked another horse, but she went right back to training normally, with no particular brilliance. Even when she galloped, she'd kind of dog it around the track. But when I put her in a race, it was like Dr. Jekyll and Mr. Hyde. She shocked me."

Before Dawn won her racing debut in mid-June by nearly seven lengths, and since then has run away with four stakes in a row. Saturday, jockey Jorge Velasquez barely had to move a muscle as the filly ran away to a 6 1/2-length victory in the $75,000 Matron Stakes.

While Before Dawn has been winning races for fillies with such authority, not a single 2-year-old colt has been showing comparable ability. Veitch has been presented with a historical opportunity -- or a fatal temptation.

Since he has been training the Calumet horses, Veitch has developed outstanding racehorses, but has also displayed one recurrent weakness: an exuberance that makes him push his horses too hard, ask too much of them too soon. There is no surer way to ruin a good thoroughbred.

Even Veitch's mother raised her eyebrows when she heard that her son was planning to run a 2-year-old filly in a stake for colts. She reminded him that his father, also a famous trainer, had done the same thing back in 1946; he won the race, but the filly was never the same afterward. "My mother told me I'm a damned fool," Veitch said.

But he was undeterred from his course of action. Veitch knows that Before Dawn will win the 2-year-old filly championship even if she were trounced by the colts, so he is in a virtual can't-lose position. He recalled that he had run his two champion fillies, Davona Dale and Our Mims, against colts unsuccessfully, but said, "It didn't do them any harm. After Davona Dale lost to colts, she won eight stakes in a row."

While he sees little risk involved in running Before Dawn against colts, he can entertain visions of glory. "This is an exceptional filly, and I'd like to give her a chance to win the Champagne," Veitch said. From what I've seen thus far, there's not a good colt around and she has a good chance to win it. If she does, and Pleasant Colony doesn't come back to win his next race, she might even be a candidate for horse of the year.

"Before Dawn has a chance to immortalize herself. You don't get a chance like that very often."