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, 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 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.