In the beginning he was a horse nobody wanted, an ill-bred, ill-tempered, ill-conformed animal who seemed destined to spend his career in the low echelons of racing.

But Saturday the 6-year-old gelding John Henry can make history. If he wins the Jockey Club Gold Cup at Belmont Park, he will displace Spectacular Bid as the top money-winning thoroughbred of all time. He also will clinch the horse of the year title, becoming the humblest horse ever to achieve that honor.

"This," says trainer Lefty Nickerson, "is the great American success story."

No student of pedigree or conformation possibly could have guessed that John Henry would win 18 stakes races and earn $2,464,510. His obscure sire, Ole Bob Bowers, was a nonentity on the track and at stud. His dam, Once Double, was even less distinguished. So there was no reason to be surprised when he sold for a paltry $1,100 at the Keeneland fall yearling sale. The man who bought him wasn't enthralled by the purchase; a few months later he put John Henry in another sale where Kentucky horse dealer Mel Snowden bought him for $2,200.

Snowden ordered the colt castrated to cure his roguish disposition, put him into training and attempted to sell him. This, it turned out, wasn't so easy. Snowden sold John Henry for $7,500, but the dissatisfied buyer returned him a couple of weeks later. Then Snowden sold John Henry again; but after the gelding was soundly beaten in a couple of $25,000 claiming races, that buyer asked if Snowden would take him back. In the spring of John Henry's 3-year-old season, Snowden finally found another owner.

Sam Rubin, a New York bicycle importer, had been a racing fan most of his life, and had long thought about buying a thoroughbred. He finally took the plunge in 1978, spending $25,000 for John Henry while entertaining no grandiose dreams. He was getting a $25,000 claiming horse, not a potential champion.

John Henry's development since then is almost unprecedented in racing history. He first showed he might be more than an ordinary claiming horse when he ran on the grass for the first time for a $35,000 price tag and won by 14 lengths. He ran in some minor stakes that year, and ran creditably. The next season, as a 4-year-old, he established himself as a solid allowance-class grass runner. That figured to be his maximum level, for thoroughbreds do not blossom into stars at the age of 5.

John Henry did.

"I don't understand it," Nickerson said. "He looks and acts like the same little horse, except that he's put on some weight. I don't think there's been a horse who's done what he's done since Stymie."

Campaigning on the East Coast for Nickerson and on the West Coast for trainer Ron McAnally, John Henry finished in the money in all 12 of his races last year and earned $925,000. This year, at the age of 6, he has been even better. He has won six of seven races, and in his last start made a dramatic stretch run to capture the Arlington Million by a fraction of an inch. That was his first victory on national television; at the age of 6, John Henry finally is getting some long-overdue recognition.

But there would be no recognition like the horse of the year title, and their pursuit of it has encouraged Rubin and Nickerson to take a gamble. John Henry could continue to amass big money by running on the grass in California, but he probably couldn't beat out Pleasant Colony in the Eclipse award balloting that way. If he beat the Kentucky Derby winner in a head-to-head confrontation, however, the title would be his.

That confrontation will occur Saturday afternoon at 1 1/2 miles on Belmont Park's main track. The conditions don't favor John Henry. Even though he did win the prestigious Santa Anita Handicap on the dirt this winter, he is a much better horse on the grass than on the main track. And he generally has been a better horse in California than he has been in New York, which is Pleasant Colony's home base. But John Henry has amply demonstrated that he is an animal capable of surmounting much more formidable obstacles than these.