Sunny's Halo defied history today. Horses who come from Canada and train in Arkansas aren't supposed to win the Kentucky Derby. Horses who have to be rehabilitated in a swimming pool aren't supposed to win the Kentucky Derby. And horses with a trainer who has no experience at the upper echelon of the sport aren't supposed to win the Derby, either.

Yet, despite all the evidence that said he couldn't win, Sunny's Halo won the 109th running of America's most famous race with ease. He scored by two lengths over Desert Wine, with the fast-closing Caveat third and the favorite Marfa fifth. Jockey Eddie Delahoussaye, who was winning his second straight Derby, controlled the race at every stage. Nevertheless, this was principally a triumph for David Cross, the 48-year-old trainer of Sunny's Halo.

Not only did he ignore most of the established precepts of Kentucky Derby training, he took a colt who once looked like a one-dimensional speedball and converted him into a tractable animal who could use his acceleration when his jockey wanted. "We've got two buttons we can push," Cross kept saying this week. Whenever Delahoussaye pushed those buttons this afternoon, Sunny's Halo gave him just the right response.

Sunny's Halo's natural quickness enabled him to break alertly and get a good position as the 20-horse field charged to the first turn. When he reached that turn Delahoussaye found himself in a position he might have dreamed of. He was in second place, just outside Total Departure, Marfa's stablemate, who was setting the pace as expected. "When I went around that turn and my horse was relaxing so good, I thought, 'We're on the way,' " Delahoussaye said. He was.

Total Departure wasn't racing anybody into defeat, as he loped along at an easy pace--a quarter-mile in 23 4/5 seconds, a half-mile in :47 1/5--then started to weaken. Delahoussaye immediately moved past him and went to the rail. Desert Wine, who had been stalking the leader, too, moved up on the outside.

For a moment Desert Wine put his head in front and, jockey Chris McCarron said, "We made the lead easy." Or so he thought. Sunny's Halo hadn't been asked to run yet and quickly regained the advantage. Yet even as the two horses battled for command, people in the crowd of 134,444 with a sense of Derby history were looking behind them, because it is horses who make a big move on the turn who traditionally win this race.

Marfa, who had had a relatively easy trip despite his No. 18 post position, started to accelerate on the turn, and wasn't displaying the tendency to bear in that had been the subject of so much prerace speculation. Play Fellow and Highland Park tried to make moves, too. But they weren't able to gain appreciably on Sunny's Halo and Desert Wine, because the leaders had been setting a relatively slow pace and still were strong. They had covered the first three-quarters of a mile in 1:11 4/5; only once in the last decade have the early stages of the Derby been run slower.

Early in the stretch, Sunny's Halo shook loose from Desert Wine and surged ahead. Marfa failed to accelerate in the last furlong--just as the pedigree experts had suspected. Only the Maryland-bred Caveat made a real run in the final furlong. "I thought we'd win," his jockey, Laffit Pincay Jr., said. "But coming to the quarter pole some horse bolted and took me way out.

Caveat never threatened the leader. Sunny's Halo reached the wire in 2:02 1/5, time that was respectable but hardly sensational over a Churchill Downs track that was very fast despite rain that started about a half-hour before the race.

Sunny's Halo earned $426,000 for his owner, Toronto stockbroker David Foster, who said his horse would run in the Preakness "for sure." Foster also said he must "sell part of him because I can't afford the insurance." He turned down $6 million for a half-interest in Sunny's Halo last week.

This was a result that hardly could have been anticipated by anyone except, perhaps, Cross, who had seen Sunny's Halo run as a 2-year-old. He won five races in Canada but was beaten every time he ventured across the border into tougher competition. In the Laurel Futurity last fall, he was trounced by 16 lengths.

But Cross knew that he was being troubled by a shin injury, and gave up almost all the rest of his training operation to concentrated his attention on him. He took Sunny's Halo to California, where the colt spent several days a week swimming to get fit without putting too much strain on his leg. Then Cross took him to Arkansas--"It's the best place for a horse to bloom"--where he won the Rebel Handicap and the Arkansas Derby. His victory in the latter was an awesome performance, but historians knew that no Arkansas Derby winner had won the Kentucky Derby. Nor had any horse won here with only two prep races as a 3-year-old since Jet Pilot in 1947.

So much for history.