Poopacabra

Poopacabra (formerly known as Ratrick) is a rat that was mutated in the sewer. He only appears in "The Preposterous Pulverizing of the Pestering Poopacabra."

The Epic Tales of Captain Underpants
Before he met George and Harold, he was in love with an albino rat that lived in the same cage as him. But one day, his love was taken away by a person. And ever since that happened, he wanted nothing more than to reunite with his loved one.

He was eventually adopted by Melvinborg and given to George and Harold for a class project. George and Harold named the little rat Ratrick. Ratrick had a love-hate relationship with the two boys. Discovering that George and Harold loved making comics, Ratrick decided that if he told George and Harold about his story so far by comic, they will help him. So, Ratrick started drawing his sad love story. But George and Harold were only impressed by the comic and didn't understand its meaning. Disappointed, Ratrick decided to run away to the sewer.

Meanwhile, Melvin was teaching his chimp how to make toxic slush. However, when Melvin drowned the slush into the sink, the slush hit Ratrick. He fell into the sewer's water. When the two mixed together, they mutated Ratrick into Poopacabra. Ever since then, Ratrick wanted revenge on George and Harold.

Poopacabra was looking all over for George and Harold to attack. However at the same time, Mr. Meaner came by to hide from Chewpacabra. Once the two saw each other, they both got to know each other's pain, and the two "acabras" became mates and finally stopped wanting revenge.