Dog

Dogs are a planned passive mob. They will serve as pets to the player.

Notch also said in a twitter post that Wolves can be tamed to produce pet dogs.

Possible Future Uses
Dogs may be able to be given commands, and possibly retrieve items for the owner. They also may be used as guard dogs, or "Hunting Partners". Another use is that a dog would be able to go where the owner commands, maybe even "Possess" the dog himself, to activate traps or lure mobs.

The dogs size is unknown so it may be able to fit though small holes. A guessed size of the dog is he would be 1 block wide, 2 blocks long, and 1 block high, enabling him to go in small single block holes, but not while he's sideways.