Robot dog

Robot dogs were robots which were fashioned in the shape of dogs. (TV: School Reunion)

Yappy was a toy robot dog available for sale at Sanderson & Grainger in Colchester in 2011. During the time that he worked there as a sales assistant, the Eleventh Doctor played with Yappy and showed it to Craig Owens. He commented that the robot dog was "not so much fun as [he] remember[ed]." (TV: Closing Time)

In 2059, Roman Groom claimed that a friend had made her domestic robot look like a dog. The Tenth Doctor, who claimed to hate how people "dress[ed]" up robots, regarded this as "different". (TV: The Waters of Mars)

In 5000, Professor Frederick Marius invented K9 Mark I while working on the asteroid K4067, describing him as "my best friend and constant companion." Marius had a dog on Earth, but weight requirements did not allow him to bring his real dog into space, so he built K9. He ultimately left K9 to the Fourth Doctor and Leela. (TV: The Invisible Enemy) The first in a series of K9 robots, K9 left the Doctor's company with Leela and ultimately regenerated into K9 Mark 2, (TV: Regeneration) while the Doctor had K9 Mark II. (TV: The Invisible Enemy) After Mark II left with Romana II, (TV: Warriors' Gate) K9 Mark III travelled with the Doctor and Adric (PROSE: Inter-Galactic Cat) before being left by the Doctor to Sarah Jane Smith. (TV: A Girl's Best Friend) By his eighth incarnation, the Doctor had seen to the construction of K9 Mark IV, (PROSE: Interference - Book Two) which the Tenth Doctor would leave to Sarah Jane after Mark III was destroyed. (TV: School Reunion)

Mickey Smith observed that he was "the tin dog", reflecting that he was to the Tenth Doctor and Rose Tyler what K9 was to the Doctor and Sarah Jane Smith. (TV: School Reunion)