Dr. Jekyll & Mangy Hide

Charlie, intending to try a brand of dog food called Happy Hound, does, only because it was tainted with a mutation chemical (thanks to Carface and Killer), he turns into a monster. Eventually, after Itchy has saved Charlie, the latter never wants to have anything to do with Happy Hound, not even to see it. So when Sasha apologizes to him for not believing the events, brings it out, and offers it, Charlie knocks it off her paws, to so much fury of Sasha that she ejects him.