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 Sarah Jane Smith. K-9 assisted Sarah Jane in defeating the Hecate worshippers of Moreton Harwood. (KAC A Girl's Best Friend)

Circa 1983, K-9 Mark III still lived with Sarah Jane, but, by chance, did not accompany her to Gallifrey. (DW: The Five Doctors)

K-9 accompanied Sarah to an archaeological dig in Egypt. (DWM: City of Devils)

In the early 1990s, K-9 assisted Sarah in investigating a supposedly haunted house. (VD: Housewarming)

In 1996, Sarah used K-9 to hack into records and find Sam Jones' home address, and later to translate Lost Boy's speech. (EDA: Interference: Book One)

K-9 was briefly possessed by a malevolent entity, but was freed when Sarah accidentally electrocuted him. (ST: The Sow in Rut)

K-9 began slowly breaking down and eventually ended up deactivated in a cardboard box in Sarah's attic. (VD: Moving On)

Hilda Winters had K-9 dismantled and analysed. (BFSJS: Mirror, Signal, Manoeuvre)

In 2007, the Doctor successfully repaired K-9 Mark III, who later sacrificed itself to stop the Krillitanes.He was once again rebuilt by the tenth doctor before his departure (DW: School Reunion)

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