It's appealing to see yourself "saving" an adult dog from a shelter but dealing with a dog with a troubled or insecure past can be very challenging.
The good news is that you'll be getting a full size dog so you know what to expect in that regard. He's likely to have at least some rudimentary training, perhaps from shelter personnel.
The worst thing that can happen is that the dog and the owner are not well matched. Inexperienced shelter personnel might place a strong-willed, albeit small, dog with an elderly couple.
Sometimes, owners over-estimate their own abilities and under-estimate the time commitment to handle dogs with tricky or troubled personalities.
Louise Louis of www.ToyBreeds.com notes that many older dogs entering a new home will not behave well. Expect problems like whining, barking, destroying things, and toilet mistakes.
That's because the dogs are anxious and do not feel safe in the new home. Do not take this behavior personally and try not to become angry.
Please recognize that the concept of gratitude is a human one. It does not exist in dogdom.
On the other hand, some owners make the opposite mistake and become too indulgent with these dogs. That will only increase the dog's insecurity and may create a dog that can't bear to be away from its owner.
If you get a second hand dog, you need to find a middle ground. The dog needs security, not pity. Be kind but be firm. You are in charge and you need to show it to your dog.
If you dog anxiously paws at your lap trying to get attention, ignore him! You make the decisions on playing, walking and feeding. Behavior problems are minimized when your dog feels secure and has a leader (you) that he is confident can protect him.
Not all shelter dogs will come with problems, but because some will, it's best to begin your relationship on the basis that problems should be expected and dealt with.
In the long run, this will help the dog's new home become it's last one.
Louise Louis is a certified canine specialist and creator of the popular website on small dogs, http://www.ToyBreeds.com
