Dog

A dog was a small mammal in the order Carnivora, native to Earth. Dogs were domesticated from wolves relatively recently by the humans. Over time, the dog has developed into hundreds of breeds with a great degree of variation. For example, heights at the withers range from just a few inches to roughly three feet, and colours range from white to black, with reds, grays, and browns occurring in a tremendous variation of patterns. Dogs, like humans, are highly social creatures, which accounts for their trainability, playfulness and ability to fit into human households.

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)

Rose was a small female dog that belonged to Jackie and Pete Tyler in an alternate version of Earth, referred to as "Pete's World" by the Tenth Doctor. Rose represented the alternate version of Rose Tyler - the daughter of Jackie and the deceased Pete Tyler in the Doctor's native universe. (DW: Rise of the Cybermen)

The dog Teazel was captured by Zygons and the warlord Brelarn used his Body Print, as dogs can go where they like. (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)

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.