The Miniature Monster is a villain from The Powerpuff Girls comic book issue, "Creature at Large!". He is an aquatic monster, born of a mutant spill, who, despite his small size, caused massive destruction to Townsville and The Powerpuff Girls.
In "Creature at Large!", a technician working on a nuclear-powered ship, slipped while carrying a barrel of toxic acid and spilled a tiny piece of toxic waste into the ocean. That one piece alone started to mutate and transform into a disgusting monstrous living creature.
The monster rose from the ocean and stomped its way over to Townsville, where it immediately rose panic among the city's civilians. People in Townsville ran around in fear as the monster's booming roar carried throughout the city streets. The Mayor of Townsville, himself was terrified of this monster and called up The Powerpuff Girls to come and save the day.
The Powerpuff Girls flew into town to see all of the collateral damage, the monster left behind on the city. Buildings were demolished, cars were overturned, and everything was on fire. The monster began to slowly approach the girls and they anticipated what kind of hideous deviant this creature would turn out to look like. When the monster finally revealed himself to them, the girls were underwhelmed by the fact that the creature was only a few inches tall. The Powerpuff Girls mocked the monster for its size and questioned how something so small could have caused all this damage. However, this monster packed a real punch, when beating the trio up and proved to them how strong it really was. The Powerpuff Girls were unable to stop it because it was so strong. However, Truck Driver Bob came by in his 18 wheeler and ran the monster over, squashing it to death.