What cartoon is about a dog eating mustard? - briefly
The cartoon you're referring to is likely "The Sneetches" by Dr. Seuss. In this story, a dog named Sylvester McMonkey McBean eats mustard as part of his plan to create chaos among the Sneetches based on their star count.
What cartoon is about a dog eating mustard? - in detail
The animated short film in question is "What's Opera, Doc?" produced by Warner Bros. and directed by Chuck Jones. This classic Looney Tunes cartoon features Bugs Bunny and Elmer Fudd in a parody of Richard Wagner's operatic masterpiece "Der Ring des Nibelungen." In the cartoon, Bugs Bunny tricks Elmer Fudd into eating mustard instead of his beloved carrots. The scene is both humorous and memorable, showcasing the clever wit that has made the Looney Tunes series a timeless classic.