Fonso (1877–1903) was an American Thoroughbred racehorse and was the winner of the 1880 Kentucky Derby under jockey . Fonso was bred in Kentucky and was a chestnut colt sired by King Alfonso out of the mare Weatherwitch. Fonso died at the age of 26 while standing as stud at the Oakwood Stud Farm in Lexington, Kentucky in 1903. His most notable offspring was the mare Fondling (br. 1886, out of Kitty Heron by Chillicothe), who produced the champion filly Imp.