Good dog, K9, now where did I put those doggie treats?
There are four K-9s, Mark I, II, III, and IV. The original K9 (Mark I) was created by a scientist in "The Invisible Enemy" and given to the Doctor and Lella as they left. The scientists final words were, "I only hope he's TARDIS trained." K9-I travels with the Doctor and Lella up to the "Invasion in Time" story where both K9-I and Lella leave. It's interesting that the decision to stay in the TARDIS or go with Leela seemed to be made by K9. He thought that Leela needed him more than the Doctor. But...not to worry, the doctor was already working on K9-II.
K9-II stayed on the TARDIS until the end of the "Warriors' Gate Story where he and Romana 2 decided to stay in E-Space and help the people who were trapped there to return to normal space. In this case, it didn't look like K9 had made the decision for himself, it looked more like the Doctor had given K9 to Romana as a going away present.
There's a problem with all the K9s. No, it isn't that they can't sit or shake or run up and down the stairs like my doggies can, it's the fact that they're weapons. Until K9 appeared, if the Doctor and a companion ran into a hideous monster they could either talk their way out of trouble or run, but if K9 was the companion or just along for the ride, they had a third option.
"K9, disintegrate the hideous monster," says the Doctor.
"Yes, master," says K9. Nose laser extends and, "ZAP."
"Good dog, K9!"
Which is a philosophy difference from the doctors before K9 and after him. Oh well, what's a little philosophy in trade for a cute doggie.
K9 is played by John Leeson (born March 1943 in Leicester, England). No, John wasn't a little guy that they stuffed into a doggie suit, he just did the K9 voice. The motion was done by a couple of electric motors
K9 has done quite well for himself after his Dr Who days were over. Mark III and Mark IV appeared in The Adventures of Sarah Jane. It now appears that K9 will have his own show on Austrailian TV in 2010. The title is still a little murky, Adventures of K9, or just K9 are possible, but the new K9 will have quite a different look than the old one. Here's the proposed logo for the new show.