Mouse-Zilla first appeared in the episode named after it, "Mouse-Zilla". It was mainly used, throughout the duration of the episode, to break into cheese reserves throughout the city and steal all of their contents, though Tobey wanted to use his creation to destroy the "really tall state building".
So far, Mouse-Zilla is the robot that has come the closest to destroying WordGirl. It managed to capture her in its extendible whiskers and would have crushed her if it weren't for Captain Huggyface stealing the remote and commanding the robot to release her.
At the end of the episode, WordGirl throws Mouse-Zilla into the city's bay. However, Mouse-Zilla survived and later made a cameo appearance in The Rise of Miss Power alongside some of Tobey's other robots.
The name Mouse-Zilla is a parody of the Japanese movie monster Godzilla.