Chosen answer:According to the episode commentary, it is literally just frame by frame editting of a sequence from the film "Anchors Aweigh", in which Gene Kelly dances with Jerry Mouse (from Tom and Jerry). They drew Stewie over Jerry in each frame, which is why he seems to stretch oddly in some parts (they had to match Jerry's movements exactly).
Chosen answer: According to the episode commentary, it is literally just frame by frame editting of a sequence from the film "Anchors Aweigh", in which Gene Kelly dances with Jerry Mouse (from Tom and Jerry). They drew Stewie over Jerry in each frame, which is why he seems to stretch oddly in some parts (they had to match Jerry's movements exactly).
Gary O'Reilly