Rovie

A rovie was a Gallifreyan mouse. Ordinarily, they possessed no more abilities than their Terran counterparts, though it was likely they were longer-lived. They could be domesticated. Some time tots kept them as pets.

The Doctor, Erimem and Shayde once encountered a rovie 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 it, it made it evolve rapidly. It developed sentience, the ability to speak, and a massive intelligence. It 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)