Cookie Chomper III was Alvin, Simon and Theodore's first pet, a stray kitten that found its way into the Seville House through an open window. Alvin wanted to name him "Chomper," Theodore wanted to name him "Cookie," and Simon wanted him to have a more sophisticated name, so they decide to call him Cookie Chomper III. Dave agrees to let them keep him and even buys a collar and tag for him to wear. Simon also helps Dave overcome his allergy to cats.
However, soon after Cookie Chomper III is hit by a car and killed. Dave helps the boys deal with their grief and embrace the memories of their brief, but happy, time with Cookie Chomper III.