Beginning in 1915, Hart starred in his own series of two-reel Western short subjects for producer Thomas Ince, which were so popular that they were supplanted by a series of feature films. Many of Hart's early films continued to play in theaters, under new titles, for another decade. In 1915 and 1916 exhibitors voted him the biggest money making star in the United States. In 1917 Hart accepted a lucrative offer from Adolph Zukor to join Famous Players-Lasky, which merged into Paramount Pictures. In the films Hart began to ride a brown and white pinto he called "Fritz". Fritz was the forerunner of later famous movie horses known by their own name, e.g., horses like Tom Mix's "Tony", Roy Rogers's "Trigger" and Clayton Moore's "Silver". In 1917, to signify "his patriotism and loyalty to Uncle Sam" it was announced to "change the name of his favorite horse from Fritz to one more truly American." He also volunteered from 1917 to 1918 with the Four Minute Men program to give short pro-war speeches across the country. Hart was now making feature films exclusively, and films like "Square Deal Sanderson" and "The Toll Gate" were popular with fans. Hart married young Hollywood actress Winifred Westover. Although their marriage was short-lived, they had one child, William S. Hart, Jr. (1922–2004).