K9 Mark I

K9 Mark I was built by Professor Marius to accompany and assist him in his work at the Bi-Al Foundation. He served as the template for all future versions of K9.

On the TARDIS
Professor Marius invented the first K9 because weight requirements did not allow him to bring his real dog from Earth into space (DW: The Invisible Enemy) though K9 may owe his existence to a temporal paradox. According to one account, the professor copied the remnants of one of the version of K9 created by the Doctor and given to Sarah Jane Smith. (DWM: Tautology)

The Professor offered K9 to the Doctor as the same weight requirements made him unable to take Mark I back to Earth with him. (DW: The Invisible Enemy) After a number of adventures with Leela and the Doctor, Mark I decided to stay on Gallifrey with his "mistress". (DW: The Invasion of Time).

On Gallifrey
There are (possibly apocryphal) accounts that this model of K9 later performed missions on behalf of the Time Lords and was given control of a space vessel called K-NEL (The Adventures of K9: K9 and the Time Trap, et al).

K9 accompanied Leela on Gallifrey for many years, but it was only after the disappearance of her Time Lord husband, Andred, that Leela and her K9 met Romana and her K9 (the two K9 units did not get on well and competed with each other). (BFG: Weapon of Choice) After Leela became Romana's bodyguard, and after many important missions on her behalf, Leela's K9 was destroyed in a bomb explosion, leaving Leela heartbroken, but refusing to allow a duplicate unit to be built in its place. (BFG: Imperiatrix)

On Earth
K9 Mark I is known to have survived the Last Great Time War, despite have been destroyed earlier. Some point later he went to the year 50'000 and met Zanthus Pia Head of the Galatic Peace Commission who he saw murdered at the hand of Ahab. Being a witness to the murder Ahab's Jixen friends attacked K9 (K9TV: The Bounty Hunter). He and the Jixen was pulled to mid-21st century Earth, Upon arrival he self-destructed both destroying himself and the Jixen. Unlike other K9 models, Mark I had by this time been equipped with a regeneration circuit which allowed him to be recreated as a new model. (K9TV: Regeneration) However, it is possible that Mark I escaped when Gallifrey was transported next to Earth. (DW: The End of Time)

Features
The mechanics of K9 Mark I included rotating ear-probes, telescopic 'eye' probe, extendable nose, flashing lights on the top and the 'eye' panel, waggling tail antennae, and ticker-tape tongue. The head could also move up and down. The shell was painted in gold/grey metallic. Around K9's neck was a tartan collar, from which hung a silver disk. (DW: The Invisible Enemy)

Behind the scenes

 * K9 Mark 1 is rumoured to return in the first episode of K9 (TV series). Bob Baker has said that the K9 model that appears in the early part of the first episode is "identical to the one Leela had in Gallifrey [sic]". This means K9 Mark 1 might have survived the Time War. In the pilot episode, he will appear as last seen in Doctor Who but will before expiring, insist his companion initiate his "regeneration" sequence, from which a new and more futuristic form of K9 Mark I will emerge.
 * Although another actor's voice is heard as K9 in the new series' promotional trailer, it was later confirmed that John Leeson would once again perform the character's choice for the TV series.