Dogs, like humans, have their own social behaviors, which can be more complex than we imagine. Dogs are naturally pack animals; they establish hierarchies and territoriality, and these traits influence their attitudes towards new dogs. Therefore, if you want to get a second dog, you must first ensure your existing dog accepts you.

If you already have more than one dog, it's best to introduce the new dog to each of them one by one. This is because if you introduce a new member to a group of dogs, the group may tend to bully the newcomer.

Let the dog have its first contact with the outside world

If you choose an environment unfamiliar to both dogs as the meeting point, the older dog is less likely to perceive the new dog as an intruder. You could consider a neighbor's yard or a park you don't frequent. If you're adopting a dog from a shelter, you can bring your older dog along and introduce them. During the process, both dogs must be on leashes.

Using positive reinforcement

From the very first meeting, you should ensure that both dogs experience positive things in each other's presence. You can start by letting them sniff each other briefly, then praise them in a pleasant and friendly tone—never in a threatening tone. Remember not to let them sniff for too long, as this can sometimes trigger an aggressive reaction.

Soon after, get their attention by giving them simple commands and treats. Next, take them for a walk and allow them to sniff and explore each other intermittently. Throughout this process, speak to them in a cheerful tone and reward them.

Dog body language

If a dog adopts a playful posture—front paws down and hindquarters raised—it means things are going well, as this is a friendly behavior towards other dogs.

Be alert for aggressive behavior, including bristling fur, bared teeth, growling, stiff gait, or prolonged staring. If you see these behaviors, immediately distract them with something else to interrupt the interaction. For example, two people walking the dogs can each give the dogs commands like "sit" or "lie down," and then give them a treat. Try again after a short while, but shorten the interaction time or increase the distance.

If the two dogs seem to tolerate each other, are not afraid of or attack each other, and their exploration of each other gradually decreases, then you can take them home.

How to get along at home

Let the dogs establish their hierarchy, and then support the higher-ranking dog, for example, by giving it a special toy or a favorite bed. Dogs have a natural hierarchy designed to maintain order, reduce conflict, and promote cooperation. Trying to determine who is superior based on personal preference will only confuse the dogs and cause more problems.

Introduce the puppy to the adult dog

Puppies often cling to adult dogs, wanting to play. Before they are four months old, puppies cannot distinguish the body language of adult dogs and do not know that the adult dog is annoyed. A good-tempered and well-socialized adult dog may bark at the puppy as a warning, telling the puppy, "Okay, stop bothering me!" Therefore, such behavior is normal and does not need to be stopped.

However, if the adult dog is not well-socialized or has a history of fighting with other dogs, it may use more aggressive behaviors to warn the puppy, such as biting it. Therefore, unless you are very sure the puppy will not be in danger, do not leave it alone with the adult dog from the beginning. Of course, don't forget to give the adult dog some quiet time and don't let the puppy bother it too much.

Expert assistance

If introducing your new and old dogs doesn't go smoothly, you should seek help from a professional behaviorist as soon as possible. Sometimes, dogs can get into serious fights, and the longer the problem is left unresolved, the more difficult it becomes to fix. Also, always remember: punishment won't improve these problems; it will only make things worse.

Source: https://www.petmily.com/behavior/introducing-your-new-dog-to-your-other-dogs/