Lassie is a fictional female Rough Collie dog and is featured in a 1938 short story by Eric Knight that was later expanded to a 1940 full-length novel, Lassie Come-Home. Knight's novel was filmed by Metro-Goldwyn-Mayer in 1943 as Lassie Come Home, with a dog named Pal playing Lassie. Pal then appeared with the stage name "Lassie" in six other MGM feature films through 1951. The "Lassie" character has appeared in radio, television, film, toys, comic books, animated series, juvenile novels, and other media. Pal's descendants continue to play Lassie today.

Source: Wikipedia