K9

There were three versions of K9 , the original was given to the Doctor by its creator Professor Marius. K9 was a mobile computer, which looked like a dog, it had a many devices (such as a stun gun). K9 proved a valuable companion to the Doctor. When K9 wanted to leave the Doctor and stay with Leela on Gallifrey, the Doctor built K9 Mark II, which was a new and improved version. Having recently recovered from decapitation by the Marshmen, K9's vital circuits were damaged by Time Winds, forcing K9 to remain in E-Space with Romana. K9 Mark III was a present from the Doctor to his former companion, Sarah Jane Smith. K9 tried to warn Sarah of the force that transported her to the Death Zone on Gallifrey. 

K9 appeared in The Invisible Enemy to The Five Doctors; also K9 and Company.