Introduce on neutral territory. At first, walk the dogs at a distance where they can see each other but are not too provoked by each other's presence. If the dogs are not showing any negative behaviors, reward them with treats just for seeing each other.
Walk both dogs in the same direction on opposite sides of a wide street. For dog-reactive dogs, you'll need more space than with dog-selective dogs. Sometimes, it's easiest to have the dog-reactive dog in back so she can keep her eyes on the new dog. Reward the dogs with treats whenever they look at each other calmly.
Walk around the available space with the dogs at maximum distance, gradually bringing them closer together until they are walking parallel to each other. You might take them for a walk around the block, maintaining safe parallel distance. It's important that you stay calm and relaxed during this process.
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. Below is info to help you decide whether this step is for you and if so, how to make it successful.
Dogs are inherently social animals that live well together in groups, but that does not mean that all dogs get along. Most dogs will welcome a new sibling, but it is not always smooth sailing.
For dogs exhibiting territorial aggression, you will need to gain enough control to have your dog sit, stay, and when calmed down, take a reward at the front door. Generally, a leash and head collar will give the fastest and most effective control (see Training Products – Head Halter Training).
Break the fight up quickly and calmly, redirect the dog's attention, take them for a walk together, and don't worry that it's going to happen again. They may or may not fight again, but if worry over it does not become your obsession, you will not be feeding them negative energy that will lead to another fight.
Aside from instincts, there are several factors that can cause a dog to display territorial behaviors. The lack of early socialization, sexual maturation, environmental factors, inbreeding, pack mentality, or an underlying medical condition can all lead to aggressive territorial behaviors.
Look for the following signs that the dogs are fighting, or that a fight may break out soon: The dog's tail looks very stiff and ridged, and any wagging is in short, quick wags. If the tail is held high while giving a stiff wag, this is even more concerning — friendly wags should look relaxed and loose.
It's never too late to socialize an adult dog. If your woof lacks experience in the world, take them outside and get started today.
Dogs that are territorial often exhibit warning, defensive and offensive behaviors such as barking, running fence lines or boundaries, charging and sometimes biting whoever or whatever 'invades' the dog's perceived territory.
Warning signs include stiffening, low growling, avoidance or hard stares. If you see these behaviors, calmly move the dogs away from each other. Remember, some dogs do not like the company of other dogs, and they should never be forced into a greeting.
The 'Rule of Three' means that you can gauge the time it might take for your dog to fully acclimate to his home in threes: three days, three weeks, and three months. Think of your new dog's first 3 days as their time to decompress as they transition from a shelter or foster home into your home.
In dogdom, there's a turn of phrase called, "Second Dog Syndrome". This describes the process of adding another dog to the home quite well, but not necessarily in a positive light. As humans, we are bound to forget all of the time and effort it takes to raise a puppy right.
Many breeders recommend that your first dog be at least one-to-two-years old before you add a second to the family. If you have an old dog, he may not be physically able to play with or tolerate a pup.