Jealousy from Established Dog
An old dog's jealousy towards the new puppy is a well-known phenomenon among owners who have attempted to introduce a new dog into a household where another dog is already established. The established pet may feel that the owner is lavishing more attention on the new pet, especially because a lot of time may be required in the training of a new puppy.
You can counteract this jealously by giving more than the usual amount of attention to the old dog during the acclimation period. Give extra treats, extra praise and extra physical contact. Let your established dog know that the new puppy isn't there to take his place.
It can help to introduce your old dog to the new puppy a few times in the two or three weeks before you bring the puppy into the home. Choose a neutral location for these meetings and, to alleviate any feelings of jealousy that might arise, allow a neutral third party to hold the puppy while your old dog introduces himself.
Territorialism from Old Dog
Dogs are territorial creatures and the introduction of a new dog into another dog's territory may result in dog fights and training issues. Your old dog may break housetraining during the acclimation period. This is his attempt to assert territorial boundaries. Don't punish your old dog too harshly for breaking housetraining; a firm 'No!' is sufficient.
Remember not to withhold attention or approval from your old dog if he breaks housetraining during the acclimation process. Your established pet needs a lot of attention at this time, as he'll be feeling insecure.
You can prepare your old dog for the introduction of a new puppy in advance before you bring the new puppy home. While still at the breeders, rub a cloth on the new puppy until it's covered with his scent. Leave this cloth lying around your house for a few days prior to the introduction of a new puppy; your established pet will pick up that scent, and won't be quite so shocked when the new dog arrives.
Even well-socialized adult dogs may become aggressive towards a new puppy. This is because they, as adults, lack the enthusiasm and energy of a puppy. Snarls, growls and even gentle bites are your old dog's way of asserting his authority and of correcting your puppy's rambunctious behavior. These aggressive behaviors are normal and should not be discouraged.
However, it's never a good idea to leave your new puppy alone in the home with the established dog. Your puppy may not understand your old dog's assertions of dominance, and this could cause the situation to escalate into an injurious dog fight if you're not there to keep things under control.