K9

K-9 was the first - and until Kamelion, the only - robotic companion to travel with the Doctor.

Biography
There were four K9s.

Mark I
See main article.

Mark II
See main article.

Mark III
K9 Mark III was built at some point by the Doctor (presumbably), which he left on Earth as a present for another companion, Sarah Jane Smith (K9 and Company). During the 1980s, K-9 Mark III still lived with Sarah Jane, but, by chance, would not accompany her to Gallifrey (DW: The Five Doctors). In early 2006, K-9 Mark III sacrificed itself to stop the Krillitanes. (DW: School Reuinion)

Mark IV
See main article.