Growling, snapping, and nipping can be just as scary as an actual bite, and they are just as inappropriate. Puppies that are confident and have been taught good manners are less likely to grow up to be aggressive dogs. But if such tendencies are not addressed by sufficient training in puppyhood, they will probably get worse as the dog gets more dominant or more fearful as it matures. Aggression may take several forms, depending on the dog's history and circumstances.
Toward the Owner
A dog that shows aggression to its owner definitely thinks it has the upper hand in the relationship. Nipping and snapping behavior that is not effectively corrected will lead to biting behavior and even though yours is a small dog, it can do plenty of damage. If it nips when you are trying to correct it or perform an activity like grooming or nail trimming and your response is to cease and desist, the puppy has learned that nipping works well as a strategy in getting its own way. Problem correction with a trainer will curb this behavior. Enforcing your leadership will also be necessary. You're back to square one in obedience — the dog needs to get your permission for everything good in its life, from eating to being petted. You'll be using the word “No” a lot, and you'll need lots of patience and restraint not to smack a dog that tries to bite the hand that feeds it. Not only is hitting a dog inhumane but in the case of a small dog, it usually makes matters worse.
Aggressive behavior is fairly common among small breeds. Dominant dogs look upon children as subordinate littermates, and they frequently inflict serious and disfiguring injuries to children, so you need a trainer's help to make life safer for all concerned. Other small dogs live in fear of kids because they perceive them as a threat to their safety.
You also need to manage the child as attentively as you do the dog. Aggressive activities like tug-of-war or teasing must not be permitted, and a child should never hug or squeeze the dog because such affection may be mistaken for physical dominance. Children are excitable, and when they run and scream, many small dogs get upset enough to bite them. Some small dogs learn to be loving housemates to children within their own family but not with strange children who visit. Simply put, managing a dog that is not trustworthy with children presents a lifelong challenge.
Small dogs are so darned cute that everyone wants to pick them up and cuddle them, but that is often a bad idea. You can be sued if your dog bites. More importantly, you do not want to be responsible for anyone getting injured, so you need to be careful about situations with strangers. You also need to begin a campaign of intense training and socialization, preferably working one-on-one with a trainer skilled in problem correction who will act the part of the stranger until your dog learns to be less fearful and defensive.
Toward Other Dogs
People tend to be amused by tiny dogs that think they are tough. It may seem comical when your Chihuahua threatens a German shepherd, but this behavior can get the little squirt in a lot of trouble. Owners who make a joke of such behavior are ignoring a real and serious danger. Laughing at such antics gives the little troublemaker positive reinforcement, actually encouraging such unacceptable behavior. Not all big dogs will take kindly to a pint-size upstart, and your little tyrant could be killed or injured if one of those big boys decided to retaliate.
It is never safe to let your pint-size pet get in the face of a larger dog it does not know. One chomp from the jaws of a rottweiler or German shepherd can spell disaster to a feisty little Yorkie or Chihuahua. Whether in your neighborhood, the park, a puppy play group, or at doggie daycare, before dogs are allowed to play together, they should be closely observed and screened to make sure they make compatible companions.
Obedience training is the best way to address this problem. If you notice your little friend “getting tall,” stiffly standing on tiptoes, it's time to quickly intervene, using the leash to pull the dog away while giving a firm “No!” It's imperative to act quickly, as one chomp from the big dog could kill your pet. Having the instructor teach you how to get the message across might someday save the life of your little Napoleon. Carefully monitoring your pint-size pet around bigger dogs will be a lifelong requirement.