In the episode "Shrinkin' in the Ray", one of Dr. Two Brains' henchmen got a new pet rabbit and named it Flopsy.
Near the start of the episode, it was revealed that the henchmen used up one of their boss's parsnip crates (which were used as fuel for Two-Brains' grow ray) while growing Flopsy with the grow ray, which the evil doctor was not happy about.
Later on, while in the mouse blimp, the unnamed henchman was sad and worried about Flopsy because he was too big to bring along on their next cheese-shrinking mission.
When a shrunken WordGirl and Captain Huggy Face were searching for clues about Dr. Two-Brains' whereabouts, The Narrator helpfully suggested a clue "the size of a really big bunny", which turned out to be Flopsy resting near Two-Brains' lair.
Flopsy was last seen at the end of the episode with WordGirl and Captain Huggy Face. It is unknown if he was ever returned to his original size afterwards.