The Giant Fish Balloon Monster (vocal effects and evil laugh provided by Kevin Michael Richardson) is a villain monster that appeared in the episode "Uh Oh Dynamo" When the monster appears from under the water he tried to ruin all the people of Townsville's fun and games the girls tried to stop but he's too strong. And he goes on a rampage by destroying Townsville. But the girls used the Dynamo to stop the fish balloon by throwing three star-shaped signs from a bowling alley that pierces its mouth, causing it to deflate and flies away uncontrollably before falling to Mount Rushmore.
The Giant Fish Balloon is a giant red puffer fish with a dinosaur-like tail and an oval body, looking like an actual balloon. He has seven (or six) eyes and large fangs. He is also covered in giant spikes that go all down his back and tail. His lip resemble that of a balloon's mouth.
True to its name, the monster can expand his body and sucks air to inflate himself, which he possesses an incredibly powerful strength in fully-inflated state. He also briefly inflate bigger to cause his spikes to explode like a sharpnel and destroying anything around him.
However, due to its inflatable-like characteristics, he is somewhat vulnerable to sharp objects, especially his lip, which revealed to be his only weak point (in regards to his rather strong body when inflated). The star-shaped bowling signs that end up on his lips easily pop holes on it, which causes him to deflate and fly away off like a balloon. This may also be a reference to deflating a balloon by cutting off or puncturing the knot, without causing it to pop.
- He is one of the only monsters, not to resign on Monster Isle.
- In the episode "The City of Frownsville", The Giant Fish Balloon was seen crying. Later in the show, he was seen laughing when Lou Gubrious slipped on a banana peel. The Girls however, never fight him in the episode.
- Although he is not a main villain, he was treated like a main villain in the episode The Powerpuff Girls Rule! where he was featured along with the rest of the main villains, trying to get the key to the city.
- His physical appearance slightly changed in the episode "The City of Frownsville." In this appearance, he was shown to have yellow eyes instead of white, his fins were brown instead of orange, and his spikes were a dark shade of orange instead of the usual white.
- A "man in a rubber suit" version of him was featured in the music video for Signal in the Sky.
- In Demashita! Powerpuff Girls Z, he makes a short cameo appearance along with the original Powerpuff in the final episode.
- The sound effect when the Fish Balloon deflates was possibly recorded directly from deflating an actual balloon, rather than a pre-made artificial noise.
- The monster's design is based on Godzilla, Gamera, Titanosaurus, and Anguirus altogether; due to it's bipedal stance, it's tail, it's spikes, it's fins, and fire breath together. The tail's crystal spikes are based on the tail crystal spikes that SpaceGodzilla has.
- Being an inflatable creature, it is somewhat ironic that the Fish Balloon instantly deflates when sharp objects hits him, but seems to be unharmed to fires and explosions resulting from the destruction. In addition, he also breathe fire.
- In reality, balloons and inflatables would usually burst if exposed to heat or fire
- Notice that the three holes from the bowling sign stars grow into one large hole the moment the monster deflates and fly away.