Twonkies are small, furry, and generally harmless critters from the comet Twonkus-3. Twonkies are only dangerous when exposed to sound of the same “color” as themselves. Then they change into something quite horrible; Stompers, Gromps and Monstros, respectively. They are possibly related to Fibs. To make matters worse, Twonkies can also evolve into three different sub-species of Elemental Twonkies - Lightning, Ice, and Dark Matter Twonkies.
Carl took one home, it evolved into a Stomper, and Jimmy Neutron had to stop the menace.