An unexpected thought, the kind that slips in when common sense goes on a quick vacation, came to jockey Angel Santiago during the Kentucky Derby this afternoon when the filly Cupecoy's Joy turned for home with 18 adolescent guys huffing and puffing in hot pursuit of whatever glory remained in this star-crossed Derby, the one we'll remember for a hundred dreams gone bad and one come true.

Bull Hancock died in 1972. His oldest son, Arthur, didn't want to run his daddy's celebrated Claiborne Farm, where for 40 years Bull bred great horses without ever winning the one race he most wanted, the Kentucky Derby. Now his son has his own place, Stone Farm, and is co-owner of this Derby winner, Gato del Sol.

"He'd sure be proud," the son said of his father. "This is the greatest thing that can happen in this life. I'm a Kentuckian. I saw how much my daddy wanted to win this whole thing."

We can love this nice turn of fate while yet saying this race was less than classic. Hillbilly hardboots here get all tingly thinking of Canonero's circle-the-field move in the 1971 Derby, and Secretariat two Mays later stopped hearts by flying six zillion miles per hour on the outside. Well, some Mays are diamonds, some Mays are coal.

Today's Derby was so mediocre that a plodder without a victory the last seven months could run wide all day and still win because--because the good horses are on crutches.

With the best of her buddies gimping around home, all grouchy at fate for keeping them out of the Derby, it might have occurred to Cupecoy's Joy that if ever a plain country girl were to dance with royalty, today was the day. It certainly popped into her rider's mind when Angel Santiago looked up and saw nothing but daylight up ahead.

"I was a little afraid by the post," said Santiago, 32, a Panamanian who once lay unconscious 12 days after falling in a race. "We had to maybe come out really sharp."

Cupecoy's Joy, who hadn't won in three months and had never won against colts, drew the No. 1 post position in a 19-horse field. From her spot on the rail, the filly had to run like crazy or else the whole equine world would be sliding down on top of her, squeezing her to the rear.

Not only did she break sharp for Santiago, Cupecoy's Joy led after a quarter-mile by 1 1/2 lengths and stayed in front, four lengths ahead halfway through the mile and a quarter race.

This is no time for idle dreaming, for halfway through a Kentucky Derby is like being halfway through a gasoline fire. Damned if it doesn't keep getting hotter. Angel Santiago had taken a peek over his shoulder going around the first turn, to see where the boys were, and now, halfway through this killing race, the guard at the gate of common sense took a nap.

"Oh, yeah," Santiago said quickly when someone asked if he thought to steal this muddled Derby by running off before anyone caught on to his work. "So much confidence I have in in the filly."

Cupecoy's Joy had the lead at little cost, really, having run the first mile in 1:37, no great shakes. Best of all, as Santiago figured it, "She was running easy all the way." Maybe, just maybe, this filly who had run more than seven furlongs only once--finishing third last month in a 1 1/16-mile stakes at tiny Latonia Race Course--could hold together down the quarter-mile stretch separating her from immortality, to say nothing of a chance to meet Howard Cosell.

Alas, alack and sad to say, it wasn't to be. The songwriter Dan Fogelberg calls the Derby "the chance of a lifetime, in a lifetime of chance," and today it was Gato del Sol who stood as the happy survivor of chance, circling the field in the far turn to run down Cupecoy's Joy.

"By the eighth pole, when I asked her to run, I knew she was tired," Santiago said.

Who passed him first?

"By the eighth pole, everybody."

El Baba, the second betting favorite, came up so brashly he bumped the filly off-stride, and Santiago took a firmer hold, slowing down the filly who by then was wobbling toward the finish, bushed by the mighty effort.

El Baba's rider, Don Brumfield, saw only Cupecoy's Joy ahead of him turning for home and said he thought, "This sucker's mine." He said, "I thought, 'I can catch that thing up in front.' "

But after they passed the filly, Brumfield and El Baba had nothing left. "He came up empty," the jockey said.

Cupecoy's Joy finished 10th, just ahead of El Baba, barely a length behind the betting favorite, Air Forbes Won, and about 13 lengths behind Gato del Sol.

"I called a buddy of mine who is a mathematician," Arthur Hancock said, explaining why he wasn't too worried about drawing the 19th post position, the last stall on the outside, "and I said, 'If we take a straight shot at the turn . . . how much ground will we lose?' He said, 'Well, the hypotenuse of the right angle triangle or something . . . ' He figured we'd only lose a foot and a half along that whole stretch."

Some Mays are diamonds.