Despite being a fictional character, Lassie is arguably the most famous dog ever.
From 1943 to 2007, Lassie was portrayed in movies first by a Collie named Pal, and by ten generations of his descendants after him.
Even though Lassie’s character is a female, there have been multiple occasions where she was played by a male Collie. This was because males have a thicker coat in summer, which makes them more “puffy.”
And to avoid human child actors outgrowing Lassie while filming, male Collies came in handy since they are bigger than females, giving the actors more room to finish the movie.