Three-year-old Thunderello, who ran second in the NAPA Breeders' Cup Sprint (gr. I), has been retired and will enter stud at Doug Arnold's Buck Pond Farm near Versailles, Ky. One of 20 stakes winners by his sire, Montbrook, Thunderello will stand for $5,000.Bred by Four Horsemen's Ranch and raced by Charles Mady, Thunderello set the pace in the Sprint before being passed by Orientate in deep stretch. He went into the race off a triumph in the Gallant Bob Handicap in 1:09.83 for six furlongs. In what turned out to be his final start, Thunderello broke the splint bone and chipped a sesamoid bone in his left front leg in the Nov. 16 Frank J. De Francis Memorial Dash Stakes (gr. I) at Laurel.Thunderello retires with three wins and three seconds from seven starts and earnings of $388,400. His dam, On the Square (by In Reality), produced two earlier stakes winners.