It is as if Farrah Fawcett-Majors developed into a great dramatic actress. Or Harold Robbins wrote the great American novel. Seattle Slew, which last year achieved fame that was largely a result of media hype, has proved himself to be a genuinely great racehorse.

Slew's triumph over Triple Crown winner Affirmed last month was brilliant. His conquest of the formidable Exceller two weeks ago was stunning. If he can best them both at a mile and a half in Saturday's Jockey Club Gold Cup at Belmont Park, he will clinch his second Horse-of-the-Year title and secure his reputation for all time.

And he will have done it all while laboring under a serious handicap. No champion racehorse in this decade has been managed so ineptly as Seattle Slew. His owners and trainer managed to conceal Slew's true ability so well that many detractors (including this one) wrongly concluded that the horse did not possess much talent. But now, with the benefit of hindsight, we can understand the reasons for his dramatic improvement in the last few weeks.

Seattle Slew exploded onto the racing scene in the fall of 1976, winning his only three starts, capturing the 2-year-old championship and showing so much promise that he frightened trainer Billy Turner. Literally.

As Slew was about to begin his 3-year-old season and his pursuit of the Triple Crown, Turner remarked, "We know that every time this horse goes to the track, what happened to Hoist The Flag can happen to him."

Hoist The Flag was the promising colt which took a bad step in a routine morning workout, shattered his leg and ended his career in a horrifying split second. Turner was determined to protect Seattle Slew from such a fate. But like an anxious parent who wants to shield his child from all dangers, he also limited his charge's potential for growth and development.

Athletes have to train hard to improve. Slew was coddled; he was raced sparingly and trained as lightly as any Triple Crown candidate ever has been. But Turner got away with his kidgloves handling of the horse, for Slew could run slowly and unspectacularly and still defeat his mediocre rivals. His time in the Belmont Stakes, 2:29 1/5, was the slowest since 1971; it would have put him 13 lengths behind Affirmed and Alydar this year. But in 1977 the performance was good enough to complete the Triple Crown sweep, albeit by virtual default.

No reservations about Slew's victories occurred to the television commentators covering the Triple Crown races. The business of television is to create stars. Seattle Slew was proclaimed a superhorse.

All the media attention seemed to afflict Slew's owners with a case of megalomania. After the Belmont Stakes, Jim Hill and Mickey Taylor horse, disregarded their trainer's advice and sent Slew to California for a race in which he was soundly trounced.

Instead of acknowledging that the trainer knew best, the owners fired Turner and replaced him with unknown Doug Peterson, who would presumably bow to their wishes. But the owners did not even seem to know what their own wishes were. They managed Slew with indecision and timidity, seemingly afraid to expose him to a defeat that would mar his reputation and reduce his future value at stud.

During the course of the season, Slew had a number of blazing workouts and won a couple of prep races - except that they were not prepping him for anything. He did not run in a stakes race until the Paterson Handicap at the Meadowlands last month, and just when he needed some hard work to become fit, Slew did not get it Jockey Jean Cruguet told the owners before the race he thought Slew was "short" - insufficiently trainer - and he was right.

So when Seattle Slew was entered against Affirmed in the Marlboro Cup, his stock had fallen to an alltime low. His old loyalists had abandoned him and the few people who had criticized his Triple Crown performances were saying. "I told you so." But with the beneficial conditioning he had gotten from a tough race only 11 days earlier, Slew was probably more fit than he had ever been in his life.

And, of course, he proved it. Slew buried Affirmed by three lengths in near-record time. That effort advanced his condition even further, enabling him to run away from Exeller by four lengths in the 1 1/4-mile Woodward Stakes. After this Saturday, he may have nothing left to do. He lost by a neck to the unheralded effectively seized control of the Dr. Patches.