William "Candy" Cummings is the most likely candidate for inventor of the curveball. Balancing the scales at 5'9", 120 pounds, Cummings wasn't exactly an intimidating figure on the mound. However, the pitch he maintained he invented as a 14 year old in 1864 made him one of the most dominant pitchers of the National Association, the forerunner to modern major league baseball.
Lacking the required tenure to join the Baseball Hall of Fame as a player alone, having played from 1872-1877, he was elected as a pioneer of the game in 1939. Many other players of the time made their own claims to the curveball, but the only one with a decent case was a player by the name of Fred Goldsmith, who did indeed make the first public exhibition of the curveball in 1870.