Maximum Security, ridden by Luis Saez, finished first Saturday in the 145th Kentucky Derby at Louisville's Churchill Downs but was disqualified.
The result of the race was contested and stewards reviewed race footage to decide on the objection.
Country House finished second and was declared the winner.
Code of Honor moved to second.
"This was my dream. I just feel so glad," Saez had told race broadcaster NBC after the apparent victory.
Saez said the horse had a good race until he got to the home stretch and the noise of the crowd of 150,729 fans unnerved the horse.
"He started getting a little bit scared," Saez said. "But then I grabbed him and I controlled him. And I kept fighting because I know he's a real fighter."
There were 19 horses in the field on a rainy day.
The track conditions were listed as sloppy.
The next race in the Triple Crown series is May 18 -- the Preakness Stakes at Pimlico Race Course in Baltimore.
The Belmont Stakes in Elmont, New York, is June 8 at Belmont Park.
