A couple of points regarding dogs:
1. Don't make any assumptions about a dog's breed. Chows never got the bad press of other breeds, but for several years running they were responsible for more emergency room visits than any other breed. They were cute little fuzz balls in pet stores, but the puppies grew into dogs, and many had very strong personalities that would not be crossed without biting.
2. Dogs usually regard their household members as pack members, and relate to them in the same way they do other dogs. A lot of behavior issues that veterinarians deal with are because people allow the dog a dominant position in the social structure. Things that we care little about and might regard as sharing, like who eats first, who gets to sit and lay where, and who goes through doorways and gates first, are seen by dogs as gestures of submission or dominance, depending on how they are handled.
3. In the same way, dogs will almost always see someone from outside their household as a foreigner to their pack. An outsider will always be regarded with at least a certain amount of suspicion. The result may be cowering, watchfulness, or outright attack, as determined by their personality and where they see themselves in their pack social structure. Their behavior can also be influenced by the behavior of the outsider - behaviors that are seen by the dog as submissive behavior or an attempt at dominance - but as mentioned by a previous post they are autonomous personalities over which no one ever has absolute control. IOW, a dog with a strong personality that feels his or her pack's territory to be challenged, or who feels his or her societal ranking challenged, may choose to attack no matter what you do.
4. When dogs are outside and running loose, their behavior changes markedly because they are with a new and different pack. The same dog that is perfectly sweet at home under the leadership of a two-legged pack leader may behave completely differently under the leadership of another pack, and may even be the leader of the other pack. Prey behavior that is not present in a stable environment with a ready food supply and a strong leader may manifest itself strongly with a new pack, even if the individuals in the pack have full bellies.