The Doctor’s robotic dog is K9. The name is a pun on “canine,” the word describing a dog, and K9 is usually written with a numeral 9 rather than the letter combination “canine.”
K9 first appeared in “The Invisible Enemy,” a 1977 serial featuring the Fourth Doctor, played by Tom Baker. He was created by Professor Marius in the 51st century and joined the Doctor after the story’s events. K9 combined encyclopaedic knowledge with a powerful laser weapon concealed in his nose.
K9 was operated as a radio-controlled prop rather than by an actor inside a costume. The design often struggled on uneven surfaces, so production crews sometimes used hidden planks or a piece of string to move him. Several K9 models appeared, and the character later became associated with Sarah Jane Smith as well as the Doctor.