The Doctor built him out of an RC car that he had shoplifted from Radio Shack. Unfortunately the prop was poorly made and prone to breaking down on set, so they often had to drag him around by wires tied to the Doctor's ankles that you could totally see.
David Tennant was known to fuck K-9 on a daily basis when K-9 was brought back in cameos such as School's A Bunion and Jenny's End. Billie Piper said of the affair, that it was "so surreal seeing David's cock stuck inside someone other than myself and Russell."
K-9 once had a plug stuck inside Sarah Jane Smith "by accident" when playing chess, and had to get it surgically removed from him, as Sarah was stuck and couldn't slip it out of herself.
K-9 later had a short-lived spin-off television series in Australia (where else, right?) called K-9 & Friends (where Sarah Jane would make vague jokes about animal abuse in front of K-9) only for it to be cancelled a month later.
K-9 never knew the answer when it was fucking important.
Still, K-9's a good dog.