K9 Mark III

K-9 Mark III was presumably built at some point by the Doctor, which he left on Earth as a present for another companion, Sarah Jane Smith (A Girl's Best Friend). Circa 1983, K-9 Mark III still lived with Sarah Jane, but, by chance, would not accompany her to Gallifrey (DW: The Five Doctors). In 2007, K-9 Mark III sacrificed itself to stop the Krillitanes (DW: School Reunion).

Behind the Scenes
Only the pilot episde of K-9 & Company, A Girl's Best Friend, ever went on air. If the series had continued, the Master would have been revealed as K-9's true creator.