Dog

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

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

A type of dog is found on Barcelona. The only known difference between them and Earth dogs is their lack of noses. (The standard joke, which the Ninth Doctor references, is "But how do they smell?"--to which the answer is "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 and the warlord Brelarn used his Body Print, as dogs could go where they liked. (NSA: Sting of the Zygons)

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.