With more than 50 candidates for the Kentucky Derby and no standouts, the race might appear indecipherable. But the Derby's history clearly suggests the type of horse who will succeed in a year like this.

The colt who will be draped with roses on May 7 will probably fit this profile:

* He had a reasonably successful 2-year-old season.

* He came into peak form in the two or three weeks before the Derby.

* He is a stretch-runner.

Until Saturday, not a single horse in America fit this description. Now there is one: Deputed Testamony, the impressive winner of the Federico Tesio Stakes at Pimlico.

Much logic underlies the success of such horses like this at Churchill Downs. To win at 1 1/4 miles so early in the season, a colt must have plenty of conditioning--what racetrack people call "bottom." But a horse who tries to cram all this preparation into the few weeks before the Derby is apt to be tailing off on the first Saturday in May. That is what happened to Air Forbes Won last year; he won all four of his career starts in March and April, but fell apart in the Derby.

A colt needs to acquire some of the necessary "bottom" as a 2-year-old. (In the Derby's 108-year history, only one winner--Apollo in 1882--didn't race at the age of two.) Then his trainer doesn't have to push him too hard early in his 3-year-old season; he can let him round into form. In a year when there is no Secretariat or Spectacular Bid who is simply more talented than any of his contemporaries, the Derby will be won by the horse who gets good at the optimal time--such as Genuine Risk, Pleasant Colony or Gato del Sol.

But the horse who peaks at the right time in an ordinary year had better be a stretch-runner. Because every owner with an ambulatory 3-year-old figures he has a shot at winning the Derby, the field will be big. And when 20 horses are in the lineup, there is usually such intense competition for the early lead that the hot pace compromises the chances of speed horses. Thus Gato del Sol came from last place in a 19-horse field last year; Pleasant Colony rallied from 17th in a 21-horse field the year before.

Deputed Testamony has the right credentials. He had an active 2-year-old campaign, winning four races, including a minor stake at the Meadowlands. He was not a superstar, to be sure, but he displayed roughly the same level of ability as Pleasant Colony and Gato del Sol did.

In the Tesio, he showed that he is just now coming into peak condition, exactly as trainer Bill Boniface had planned. Swooping around the field, he drew away in the stretch to score a decisive 2 1/4-length victory. He ran faster than older stakes horses like Eurodancer and Northrop did on the same afternoon.

The time of the race was so fast that it looks as if it could be a fluke. If the winner ran so fast, then the five horses who finished within seven lengths of him all ran the races of their lives, too. Subsequent performance by these horses will indicate whether that 1:42 4/5 clocking for 1 1/16 miles was an aberration, a result of some sudden change in the inherent speed of the track. But if the time can be taken at face value, it suggests that Deputed Testamony is four lengths better than Marfa, the California colt who is widely considered the Derby favorite.

In addition to the doubts about the time of his Tesio victory, there are some things about Deputed Testamony that don't fit the classic Derby profile. The Derby is seldom won by trainers or riders who haven't previously competed in the race; neither Boniface nor jockey Herb McCauley has been through this high-pressure experience before. Nor is the Derby won very often by colts with utterly nondescript breeding.

Moreover, it is a little hard to envision a dumb name like Deputed Testamony appearing on the Kentucky Derby souvenir mint julep glasses for the rest of history. Even so, this implausible colt has suddenly become a legitimate contender--maybe even the strongest contender--to win America's most famous race.