From "Snoopy Come Home." After getting kicked out of the Library, Snoopy gets all pissy and goes and beats up Linus and then Lucy. The boxing scene is one of my favorite scenes of all time- from any movie.
Edit 7/7/2023- Really apologize for the potato quality of the video. This was uploaded before some of you were born, and before it was easy for me to capture HD video. The quality is so poor, in fact, that I think it evades copyrighted material detection or something. I mean, I have the DVD of Snoopy Come Home now, but it would probably be claimed instantaneously.