Fonso (1877–1903) was an American Thoroughbred racehorse and was the winner of the 1880 Kentucky Derby. Fonso was bred in Kentucky and was a chestnut colt sired by King Alfonso out of the mare Weatherwitch.Fonso won the Phoenix Stakes as a three-year-old over Luke Blackburn who finished third, but is best remembered for going on to beat the favored Kimball in the 1880 Kentucky Derby.