In the final and most prestigious event of the Washington International Horse Show last night at Capital Centre, Katie Monahan Prudent finally came out a winner.

The Middleburg, Va., resident rode Special Envoy to the best finish in the $40,000 Michelob President's Cup Grand Prix in 30.42 seconds to beat 38 other entries, including two jumpoff qualifiers.

Earlier this week, Monahan Prudent, 33, was second in two international jumper events and third with husband Henri Prudent (riding Pensato) in a costume pairs class.

"He's {Special Envoy} young, only 9, and German and has a feisty temperament," said Monahan Prudent. "He was in a slump midyear, but is definitely out of it now."

With the win, Monahan Prudent, 1986 rider of the year, clinched the show circuit's national rider title with 25 points. Before last night's competition, she was in second place behind Michaela Murphy of Far Hills, N.J., who is second with 20 points.

Murphy rode Gusty Monroe to a 32.25-second finish in the President's Cup jumpoff for the runner-up ribbon.

Rodney Jenkins of Rapidan, Va., tried for his 69th career grand prix victory, finishing the jumpoff in a faster 29.33, but his horse, Monroe, knocked over the final fence in the six-fence course to gain four faults and finish third.

Four other entries tied for fourth, including Greg Best of Flemington, N.J., on Gem Twist, with four faults in the second round. Best, 23, is the rookie of the year with more than $90,000 in winnings this year. He has broken every rookie record on the grand prix circuit in his four victories this year.

The other fourth place finishers were Jeffrey Welles on Byron, Norman Dello Joio on Aga Khan and Michel Robert on Pequignet Lafayette.

Just 12 of the 39 entries scored clear first rounds to advance in the competition. Monahan Prudent, Murphy and Jenkins had faultless second rounds through a difficult 10-fence course to compete in the timed jumpoff.

"The course was brilliant with the distances, fence placement . . . {it} was very difficult," Monahan Prudent said.

First place in the President's Cup was worth $12,000, second place $8,000 and third place $6,000.

The French equestrian team was the top team in the show with 70 points. The United States was second with 39. Robert scored 17 points this week, winning two jumper events, to gain the show's international rider championship.

Dance On Ice, ridden by Katie Huber of Great Falls, won the pony hunter grand championship and medium working pony hunter championship with 10 points.

Hillary Schlusemeyer rode Hearts Of Fire to the small pony hunter championship with 10 points. Her mount, Thyme, was the reserve in that category with six points.

Lego, owned by Charles Ziff of Dallas, and ridden by his son David, won the $10,000 Junior Jumper Stake in a jumpoff.