It can take up to one month for an old dog and new dog to really settle in and accept each other's position in the pack.
If you want a second dog, you need to be ready to commit to this process and not panic.
Set reasonable goals when you bring a new dog into your pack.
Remember and respect that your resident dog and/or cat may perceive the new dog to be encroaching on their established territory, which can be very stressful.
Proceed slowly and calmly.
Slow-paced introductions may help prevent any fear-based or aggressive reactions from developing. If bad behaviors are not reined in from the start, they can become habit and be very hard to change in the future.
Never leave new pets unattended, even if a pet is caged.
When two pets meet, it is imperative you watch them at all times.
The situation can change suddenly.
If you have more than one resident dog, introduce each dog one at a time to the new dog to prevent them from overwhelming the newcomer.
Stay in control of the introduction.
If you are not sure how your pet will react, take the necessary precautions to keep him (and you) safe.
Be patient and adaptable. You will need to teach your new dog to trust you while communicating to your resident pets that you will continue to keep them safe. Building good relationships takes time.
Dog to Dog
Before you bring the new dog (or puppy) home, bring home his scent so your resident pets can be introduced to his smell first.Rub the new dog with a cloth or use a blanket he has slept on and bring it into your home and place it where he will be sleeping.
Introduce in a Neutral Location
Introduce the dogs in a neutral location that is unfamiliar to both dogs, such as a park. This prevents your resident dog from feeling his territory is being threatened.
Each dog should be on a loosely held six-foot leash and handled by a separate person.
Try to stay relaxed so the dogs don’t pick up on any tension you might be feeling.
Don’t force an interaction between the dogs.
Just walk near each other for a few minutes. One or both of the dogs may ignore each other, which is fine. Just stay upbeat and give the dogs time to get comfortable with the situation.
Allow the dogs to sniff each other for just a few seconds, with the handlers offering high-pitched, happy praise if there are positive signs from the dogs. Then lead the dogs away from each other. Do several more sets of brief introductions, which prevent the dogs from focusing too hard and escalating to an aggressive response.
Refocus each dog’s attention with obedience commands or short walks.
There are two goals with this exercise:
Watch the dogs’ body language.
Things are going well and you can proceed to the next step if you see:
Loose body movements and muscles
Relaxed open mouths
Play bows or other playful posturing
However, take caution if you see:
Stiff, slow body movements
Hair standing up on the back
Tensed mouth or teeth-baring
If you see any of these types of reactions, quickly lead the dogs away from each other and try to get them to focus on you.
Then you can try a very brief introduction again, at a further distance. Only proceed to the next step when you see the dogs are tolerating each other.
Managing the New Dog in Your Home
Pick up all pet toys, food bowls, beds and the like before you bring the new dog into the house to prevent any tiffs over prized possessions.
You can return the resident dog’s toys to him in a few weeks, and give the new dogs some new toys of his own.
In the meantime, give the dogs toys only when they are in separate areas or their crates.
When you bring the new dog home, put your current pets in a separate area of your home; then, walk the new dog around your home on a leash to show him where he will sleep and eat, where the other pets sleep and eat, etc.
Establish boundaries in your home by using baby gates and closing off rooms and areas while all the pets acclimate to the new situation. This way they can see and get used to one another. Allow the resident dog to roam the house, while confining the new dog behind a barrier at first.
Keep the resident dog’s areas for sleeping and eating separate so he doesn’t feel his territory is being threatened.
Feed the dogs in separate areas, and pick up their food bowls after feeding time is done.
Keep the dogs confined in separate areas of your home any time you are away or can’t watch them.
While your dogs may enjoy each other as playmates, supervise their play to prevent them from getting over excited, which can lead to injury of one or both dogs.
Interrupt their play if one dog begins to bully or growl at the other, and separate them for a few minutes.
Praise them when they are playing well together.
Remember to devote plenty of time to each dog individually for both training and play. If one dog is much older or less energetic than the other, be sure you give him time and space to himself so he can rest and feel secure.
Puppies to Adult Dogs
Not all resident dogs will welcome a new puppy into the home.
Puppies are notorious for looking for attention from adult dogs (and everyone else), and so must be supervised when they are with other animals. Very young pups may not pick up on an adult dog’s body language that says he’s had enough playing.
A well-socialized adult dog may growl to tell the pup to back off, which is appropriate behavior that helps the puppy learn boundaries.
However, an adult dog with poor social skills may present a danger to the puppy, as he may only know to bite rather than growl. Thus, do not leave the dog and puppy alone together.
Respect the adult dog’s need for puppy-free quiet time, and be sure to spend one-on-one time with him as well.
Handle the puppy-to-dog introduction as you would between dogs.
Keep both animals leashed, carefully watch their body language, allow brief sniffs, and offer praise when they behave well.
Know When to Get Help
People keep household pets because they enjoy their antics and companionship. However, if your dog doesn’t get along with other pets, this only creates tension and disharmony in your home.
A qualified dog trainer such or Dog Behavioral Therapist can help resolve conflicts your dog may be having with other pets, and can provide ways to help you live in a peaceful, happy household of pets and people.