Many casual racing fans have not heard of him yet, but the best thoroughbred in America this year may be a colt named Fappiano.

The 4-year-old showed how good he can be on Memorial Day, when he won the Metropolitan Handicap and missed Belmont Park's track record for a mile by one-fifth of a second. The performance left student of speed handicapping agog. On Saturday he can cement his status if he wins the prestigious Suburban Handicap here at Belmont Park.

He is facing two rivals with lofty reputations, Temperence Hill and Winter's Tale, as well as Belmont Stakes runner-up Highland Blade, but he figures to crush them all.

Fappiano showed immense promise from the moment he first stepped onto a race track, winning all four of his races as a 2-year-old and setting one track record. But rarely has such a precocious horse been handled so patiently and painstakingly. Jan Nerud learned his profession from his father John, a conservative old-school horseman who trained Dr. Fager and other champions. He not only resisted the temptation to aim Fappiano for the Triple Crown series, but never even asked the colt to run as far as a mile until August of his 3-year-old season.

Nerud had some doubts about Fappiano's stamina and said, "He needed a bit of racing before he could go a mile. We had to spend a lot of time teaching him to rate, to keep calm, to get in the groove of coming off the pace."

Fappiano still lost his first two distance races last year, although the failures were slightly deceptive he had the misfortune to keep running into horses who ran the races of their lives when he met them. But at the age of 4, in the Metropolitan, he finally translated all his potential into one incredible performance.

Fappiano whipped a superb file by nearly three lengths and ran a mile in 1:33 4/5, which was extraordinary because the track was rather slow that day. On a reasonably fast strip, he might hae shattered the track record and threatened the world record. Great horses like Affirmed, Seattle Slew and Spectacular Bid never ran a figure like that in their lives.

On Saturday Fappiano attempts to run 1 1/4 miles for the first time and Nerud concedes, "You're always guessing until a horse shows he can do it." But the distance is his only obstacle; if he can handle it even passably well, horses like Winter's Tale and Highland Blade won't be able to touch him.

Winter's Tale was the best horse based in New York last year, the winner of several major stakes, but he hasn't been the same animal in 1981. He has raced three times and finished second three times, and while trainer Mack Miller insists that nothing is wrong, there is no evidence to suggest that he is about to recapture his best form.

If Winter's Tale appears to be a has-been, Temperence Hill is a never-was. True, he won more money in 1980 than an thoroughbred in history ever has won in a single season, and was voted the country's champion 3-year-old. But he accomplished these feats more through luck than ability. Everything always went his way. Somehow he managed to earn more than $1 million without ever beating a ton-class horse or overcoming even mild adversity.

In the Suburban he must carry 127 pounds and spot seven pounds to a vastly superior horse. Fappiano ought to expose Temperence Hill's shortcomings mercilessly, and everybody else's as well.