K9

K-9 was the first - and until Kamelion, the only - robotic companion.

In fact, there were three K9s. K9 Mark I was built by Professor Marius to accompany and assist him in his work at the Bi-Al Foundation. This K9 joined the Doctor and Leela, ("The Invisible Enemy"), but left with Leela when she decided to remain on Gallifrey with her new lover, Andred ("The Invasion of Time").

A K9 (Mark II) ("The Invasion of Time"), constructed by persons unknown and more or less identical to the original, which accompanied the Doctor, and later Romana ("The Ribos Operation") Romana. Damaged by the time winds, K-9 Mark II could not cross back into N-Space with the Doctor, so decided to stay behind at the Gateway with Romana, to help her free the remaining Tharils from slavery ("Warrior's Gate").

The Doctor (presumbably) at some point built K9 Mark III, which he left on Earth as a present for another companion, Sarah Jane Smith (K9 and Company). At a later date, K-9 Mark III still lived with Sarah Jane, but, by chance, would not accompany her to Gallifrey ("The Five Doctors").