Mitzi was an Earth cat that had been sent into hyperspace by human scientists.
She landed on a planet that had been terraformed. When humans came to inhabit it, Mitzi took control over them and forced them to serve her. Ninety years later, the Tenth Doctor and Rose Tyler landed on the planet that she ruled.
The Doctor confronted her and persuaded her to go and step down. Mitzi then took a trip in the TARDIS and was sent to the Powell Estate. Rose told the Doctor that she found Mitzi and renamed her in her past. She would have lived for another five years under the Tyler family's care. (DWA: The Cat Came Back)
When the the Ninth Doctor was investigating the Autons, Rose Tyler told him the reason she had a cat flap was that she used to have a cat. (DW: Rose) Rose later remarked she had a cat like the ginger one she befriended in 2012. (DW: Fear Her)