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 portrayal of Lassie bears some features in common with another fictional female collie of the same name, featured in the British writer Elizabeth Gaskell's 1859 short story "The Half Brothers". In "The Half Brothers", Lassie is loved only by her young master and guides the adults back to where two boys are lost in a snowstorm.
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. Pal's owner and trainer, Rudd Weatherwax, then acquired the Lassie name and trademark from MGM and appeared with Pal (as "Lassie") at rodeos, fairs, and similar events across America in the early 1950s. In 1954, the television series Lassie debuted and, over the next 19 years, a succession of Pal's descendants appeared on the series. 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