The 2000 Kentucky Derby winner was Fusaichi Pegasus. He was ridden by jockey Kent Desormeaux and trained by Neil Drysdale. Fusaichi Pegasus became the first favorite to win the Derby since Spectacular Bid in 1979.
The 2000 Kentucky Derby winner was Fusaichi Pegasus. He was ridden by jockey Kent Desormeaux and trained by Neil Drysdale. Fusaichi Pegasus became the first favorite to win the Derby since Spectacular Bid in 1979.

Aptitude was second to Fusaichi Pegasus in the 2000 Kentucky Derby.

'FuPeg' is the nickname of Fusaichi Pegasus [foaled April 12, 1997]. He won the 2007 Derby, with Kent Desormeaux [b. February 27, 1970] as his jockey. His unusual name honors his owner Fusao Sekiguchi [b. December 20, 1935], lays claim to being '#1' or 'the best' ['-ichi'], and memorializes the winged horse of ancient Greek mythology.

I will assume by "derby" you mean Kentucky Derby. I will also assume that you only mean winners of the Derby. There have been two Derby-winning horses with 15 letters in their names: Foolish Pleasure (1975) and Fusaichi Pegasus (2000).

Aptitude was the second place winner in the 2000 Kentucky Derby. He finished 1-1/2 lengths behind the winner, Fusaichi Pegasus. He finished 4 lengths ahead of third place winner Impeachment.