Twonkies cough out multiple babies from their mouth (Up to about 30 at a time), and grow quickly--roughly 1 day to become an adult and to reproduce. Stompers and Gromps do not seem to be able to reproduce, however (thus heavily reducing their population).
Reaction to external sounds
Twonkies are agitated by the sound of dulcet music to the point they will mutate and become agressive. If they hear music in any variety that is remotely pleasant, Twonkies will start to cough, their eyes turn black, and they evolve into a bigger twonkie and attack the source of music (Carl doing his bending and stretching song, Hugh's puppet, Libby's radio at babysitting class, Miss Fowl's ABC song and Nick's opera song). If multiple Stompers are gathered together, they can merge and evolve into an adult Twonkie. However, music that is utterly horrendous and cacophonous will lull them to sleep. This kind of sound can also stop them from evolving.
Twonkies can bounce around walls, making them almost indestructible. They can camouflage themselves to any color, and have "excellent motor skills". Large evolved Twonkies are durable enough to even survive an ion blast, and many other rays (A force as much as several rocket combustion engines). Gromps are also capable of breaking through reinforced steel. Twonkies don't seem to be very intelligent, and Stompers and adults are even less intelligent. When they get upset, they pound the ground with their fists and roar. Twonkies are able to sleep, and their skin color varies from blue to green to green with black dots. Their weakness is Sheen's singing, which they have an inability to resist falling asleep to because of how disturbingly tone deaf his voice is. In the video game, the music makes them devolve.
Twonkies are evolutionary creatures. They can evolve into Stompers, which have sharp teeth, spiny skin, and claws. Stompers behave very mischievously and will wreak havoc in every way imaginable due to their naturally impish dispositions. If several Stompers are allowed to unite, they can coalesce into a Gromp, an even more vicious adult form with searing red eyes and a gaping mouth adorned with several rows of sharklike fangs that will savagely attack anything and anyone that enrages it. In the video game Stompers evolve into a juvenile version of the Gromps that looks like a gradual transition into a bigger adult form. It appears that a Twonkie will evolve into this form on its own if given the chance, but as a group, they can bypass this evolutionary stage by joining as one. If a Twonkie, Stomper or premature Gromp hears more music, they evolve into the next stage.
- In many episodes, Twonkies can be seen in various places, possibly as a reference that Jimmy forgot to send Sheen’s Twonkie back and it produced offspring as a result.
- The Twonkies are parodies of the Mogwais and Gremlins from the 1984 movie "Gremlins" and its 1990 sequel. Just as the Twonkies change into monsters, once the Mogwais eat after midnight they turn into Gremlins. They may also be parodies of Furbies but this is not confirmed.
- In "My Big Fat Spy Wedding" a Twonkie is seen listening to the song "I Have the Ring" but strangely did not evolve. However, due to the song being ridiculously over-the-top and corny, the Twonkie may have actually liked listening to it.
- Ironically there was a sci-fi monster movie called that featured a monster called a Twonkie which was a mechanical amalgam of mechanical appliances.
- Their vocal effects are done by Frank Welker, who also did the vocal effects for Goddard and he also did the voice of "Stripe" from Gremlins.
- In the "League Of Villains" a twonky that looked exactly the same as the one Sheen had was left 75 million years in the past with the villains and dinosaur.
- Every time a Twonkie transforms, its evolution is not entirely shown.