the only ones that I know of that guarantee you a mega snack at elder are couch potatoes and evil magma peas. I have been farming for couch potatoes. you can also farm fro evil magma peas at various bosses, never tried though.
Ok now this is EASY. Couch Potatoes. They may take a while, but they give you mega pet snacks, 1 per each when they are done. They only get rank one in pests, and they only need music and water each day.