Rovie

A rovie was a Gallifreyan mouse. Ordinarily, they possessed no more abilities than their Terran counterparts. However, the Doctor, Erimem and Shayde once encountered one that had become lost in the Doctor's TARDIS and subjected to the raw power of the Time Vortex. Though such exposure should have killed the mouse, it instead forced it to rapidly evolve. It developed sentience, the ability to speak, and a massive intelligence, and then tried to take over the TARDIS. With Shayde's assistance, the Doctor managed to disrupt the rovie's control of the TARDIS. Shayde then took it back to Gallifrey for further study. (BFA: No Place Like Home)