Frank Thrall Weatherwax (June 23, 1902 – December 17, 1991) was an American actor and animal trainer. He is best remembered with his brother Rudd Weatherwax for their famous collie, Pal, the dog who became famous as Lassie in the 1943 Metro-Goldwyn-Mayer film Lassie Come Home. He also trained the dog, Lightning, for the movie A Dog of Flanders, 1935 version. The WIKIpedia article may be wrong when it says, "He also trained the dog, Lightning, for the movie A Dog of Flanders, 1935 version." I have no idea if he worked on the 1935 version, however, Spike, who played the title role in "Old Yeller", played the title role in the 1960 version of "A Dog of Flanders" with Donald Crisp and David Ladd.
Weatherwax was born in Camp Palomas, New Mexico Territory, the son of Anna Elisa (née Wallis) and Walter Smiley Weatherwax. Weatherwax trained the original Asta for The Thin Man, a film produced in 1934. This movie had five sequels and an accompanying television show. Weatherwax's Asta was not believed to star in any of these. He also trained Spike, who appeared in Disney's Old Yeller and in other film and television programs. Weatherwax played a bully in Little Lord Fauntleroy in 1921 with Mary Pickford. In 1920 he appeared in the silent film version of Heidi and in The Jucklins.