Excerpt  from Dog Training: A Lifelong Guide Training Your Dog

If you’re looking for a shelter dog to be a playmate for your loyal family dog, an introduction on neutral turf is essential before you sign those adoption papers.

Bring your family dog to the shelter and make the introduction outside in an open area. Take hold of the leash attached to your dog and have a friend or shelter worker hold the leash of the potential adoptee dog. Initially, keep plenty of space between each dog. Don’t walk them directly toward each other. Rather, walk them in parallel paths, slowly merging into a “V” until they are within butt-sniffing range of each other (this is the way dogs introduce themselves). Keep loose leads as they check each other out, but don’t let the leashes tangle. If they behave, bring both dogs indoors, always letting the family dog walk into the room first. This serves as a key signal to the shelter dog that, at least for now, the family dog ranks higher in the pack.

Sit down and quietly watch the two dogs interact. Some dogs bond within seconds, minutes, or hours. Others take weeks, even months. How quick and how strong the friendship bond develops depends on the two dogs. Don’t assume or expect love at first sight. Quite often, the best doggy relationships begin with the family dog either growling or snubbing and ignoring the prospective dog. The family dog should not be disciplined, punished, or yelled at for growling at the prospective dog. The best relationships are ones that have a clear distinction in rank hierarchy: A crabby family dog, along with a prospective dog who accepts his lower ranking, can be the best introduction.

Don’t be offended if the two dogs get lost in playing together and temporarily forget about you. That’s also a good sign. However, be a little cautious if the shelter dog immediately starts to push, nudge, snarl, or growl in an attempt to get your family dog away from you. This scenario is acceptable only if the family dog asserts himself and the prospective dog acquiesces. Unless, of course, you have a “weenie” dog and are looking to adopt a strong, leader-type. Otherwise, you’re witnessing a duel for the title of dominant dog of the household.

Finally, once you’ve adopted the new dog and brought him home, give your family dog some slack in his behavior. Don’t interfere if the family dog growls, snarls, or even charges (without biting or scratching) the newcomer. The family dog is showing rank and dominance, which is normal and good. Don’t yell at or discipline your family dog during this I’m-the-boss display. You’ll deliver the wrong message to both dogs. Your family dog may interpret this as you wanting the new dog to rule. The new dog may think you want him to step up his role and exercise more dominance. As a consequence, confusion and aggression may arise between the two dogs, which, without human interference, they often work out fairly well themselves.