Idun (1955-1979) was an American Thoroughbred Champion racehorse who won U.S. Champion 2 and 3-year-old filly honors. Sired by Royal Charger, a son of the very important sire Nearco, Idun was owned by , wife of Charles Ulrick Bay, business executive and the United States Ambassador to Norway from 1946 to 1953. The filly was ridden by future Hall of Fame jockey Bill Hartack and trained by another future Hall of Famer, Sherrill W. Ward who would later condition Forego.