Dog

A dog was a medium-sized, quadrupedal mammal native to Earth. Dogs were domesticated by humans who frequently kept them as pets. They came in different breeds of all different sizes.

The robot designated K9 was made in the form of a dog, but had advanced technology and human-level intelligence. (DW: The Invisible Enemy)

A type of dog was found on the planet Barcelona. The only known difference between them and Earth dogs was their lack of noses. (DW: The Parting of the Ways)

In Pete's World, Rose was a small female dog that belonged to Jackie and Pete Tyler. (DW: Rise of the Cybermen)

The dog Teazel, who belonged to Edward Lunn, was captured by Zygons in September 1909. The warlord Brelarn used his body-print to assume Teazel's form, as dogs could go where they liked. He was therefore able to spy on the Tenth Doctor, Martha Jones, and others. The real Teazel was eventually rescued from the Zygon ship. (NSA: Sting of the Zygons)

The Bandogge was a type of mongrel dog. (DW: The Shakespeare Code)

In The Year That Never Was, wild dogs roamed the streets of London. (DW: Last of the Time Lords)

The Cybus Cybermen converted dogs into two types of Cybernetic servants, Cyberdogs and Cybershades. (DWBIT: Enemy Mine, DW: The Next Doctor)

Prisoner Zero took the form of Barney Collins while the real Barney was in a coma. Barney was dreaming about walking his Rottweiler, so Prisoner Zero's "Barney" form had his dog on a leash. As Prisoner Zero was prone to losing track of which of its stolen form's mouths to use, the "dog's" growling came from "Barney's" mouth. (DW: The Eleventh Hour)

During the 22nd century Dalek invasion, packs of dogs were mentioned as roaming the woods and were said to hunt travellers. (DW: The Dalek Invasion of Earth)

On Gemino, the First Doctor, John and Gillian discovered a dog in the ruins of Gemino City. They followed it back to its master, one of the survivors of the war between Gemino and Gemina. (TVC: The Secret of Gemino)

Jim Purcell owned a dog named Bernard who watched as he was absorbed by the carpet into the doll's house. (DW: Night Terrors)

Behind the scenes
During the filming of The Ribos Operation, Tom Baker was bitten by Paul Seed's Jack Russell terrier when he tried to have it perform a trick Seed himself had done. This left him with a deep cut to his upper lip. As a result, publicity stills for The Ribos Operation saw him wearing a sticking plaster on his face, and for the remainder of filming Tom Baker required a heavy and uncomfortable makeup application to cover the wound. To lessen the necessary makeup coverage and explain the healing wound on-screen, a scene was added to the beginning of the following story (The Pirate Planet) wherein the Doctor struck his lip on the TARDIS console. The scar remained visible for much of Season 16. (DCOM: The Pirate Planet)