K9 is a fictional male Robot in the Doctor Who series. His allegiance / loyalty is to Third Doctor. There have been three K9's, the Robot companion of the Doctor. The third K9 was given to Sarah Jane Smith, on Earth, by the Doctor, as a present. Although we don't know which Doctor it was, you can only imagine that it might have been the 4th Doctor. The first series with David Tennant at the helm, there is one episode that I know of, which features Sarah Jane Smith and K9 mark III. There are no other K9s. The robot companion is to be killed off for good allegedly.
Although the Doctor never uses a laser or a gun, the K-9 unit's nose is equipped with a laser which its not afraid to use. The K-9 unit is an intelligent communicative machine that runs on wheels rather than on legs like real dogs.