K9 was the robotic companion of the Fourth Doctor (played by
Tom Baker) and lasted from the 15th to the 18th seasons of
Doctor Who. Basically a metal box (slightly larger than a breadbox) on hidden wheels with a dog shaped head and a waggable tail. He had a small barrel beam weapon (of varying power) that came out of his nose and the ability to scan people and computers with his spinning radar dish ears and an extensible rubber sucker (between his eyes). His voice was provided by John Leeson (though David Brierly also did it for a brief spell).
While he may have been beloved of the audiences of Doctor Who, K9 was not at all popular with the script writers. Due to his sensors, weapons, etc. they either had to ignore his abilities or lose him to allow tension in the stories. He ran low on energy so often you'd have though he was using a couple of AA batteries.
His being a radio controlled model in a TV studio (not known for their lack of radio noise) meant he was fairly unreliable and often either refused to move or ran off in a random direction. In the show he seemed to teleport from location to location when there were no flat surfaces around.
He first appeared in "The Invisible Enemy" as the robotic companion of Doctor Marinus (who gave him to the Doctor at the end of the story). The script writers thought they had got rid of him when he stayed with Leela (played by Louise Jameson) when she left the Doctor. However the Doctor created K9 Mk II (as he was so popular).
Mark II was finally disposed of when the second Romana (played by Lalla Ward) departed the series in the story "Warrior's Gate". He was so damaged he could only survive by staying in E-Space with Romana.
I believe it is K9 Mk III that appears in the one-shot "K9 and Company". This is interesting for the fact that he appears with Sarah Jane Smith (played by Elizabeth Sladen) -- a companion of the Doctor's from before K9's time on the series.