Dog

A dog was a medium sized mammal native to Earth. Dogs were domesticated by humans who frequently kept them as pets.

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

A type of dog was found on Barcelona. The only known difference between them and Earth dogs was their lack of noses. A joke developed around this; "but how do they smell?"--to which the answer was "Terrible!" (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 was captured by Zygons/ The warlord Brelarn used his Body Print, as dogs could go where they liked. (NSA: Sting of the Zygons)

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

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 and his Rottweiler. This was because the real Barney Collins, in a coma, was dreaming about walking his dog. (DW: The Eleventh Hour)

During the 22nd century Dalek invasion, packs of dogs roamed the woods. (DW: The Dalek Invasion of Earth)

Behind the scenes
During the filming of The Ribos Operation, Tom Baker was bitten by Paul Seed's dog 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 plaster cast on his face.