The First Rani created monstrous mice in an experiment on Gallifrey. These mice attacked the Lord President and his cat, injuring the former and completely eating the latter. (TV: The Mark of the Rani) The Time Lord messenger also indicated that he lost his cat in this incident. (AUDIO: The Man Who Wasn't There) The Seventh Doctor recalled that the Rani and her giant mouse came to his graduation party. (COMIC: Party Animals)
The creation of monstrous mice on Gallifrey dated back to Rassilon, who made horrors with the Loom of Rassilon's Mouse. (PROSE: Happy Endings)
As a tutor in the War in Heaven, the Rani attached an artron emitter to a normal rat, allowing it to fry a gallifreyan cat with its mind. (PROSE: The Taking of Planet 5)