When introducing your new puppy to your other dog; Once you have purchased your new puppy and are ready to bring it home do not take the puppy into the house, let the 2 of them meet on neutral ground. Either bring your other pet in the car to pick up the puppy to enjoy the ride home together or in a part of the yard that your dog does not go to. This will help your other pet to be more open to the transition and not go into protection mode.