Why is my dog aggressive toward our new dog? How do I introduce them?

Separate them for a few days then gradually reintroduce them. First with a door between them so they can hear and smell one another then separated so they can see but not reach one another and finally a supervised physical introduction. Use leash, collars and muzzles if necessary. Pheromone products such as the Adaptil collar or DAP plug in diffuser may decrease any anxiety or aggression. Provide adequate numbers of bowls, beds and toys to prevent food aggression or resource protection.

If your dog does not get along with other dogs it was probably a very bad idea (and a very stressful one) to bring a new puppy into the house. A new puppy can also be hard around a newborn as they aren't yet sure how to interact with each other. You can try to continue to get the two to get along by starting with limited introductions that are supervised, however if a fight breaks out the interaction should be stopped immediately and started again later. Never leave the two unsupervised together as even a good play sessions can cause accidental injury. You can also use baby gates as a way for the two to "meet" each other without being able to physically be together. If fights are continually breaking out, talking to a trainer or behaviorist that can observe within the home is best.

Introduce them to one another gradually. First, by allowing them to hear and smell one another through a closed door, then by sight without physical contact, then supervised contact. You will need to progress gradually through each phase. Correct any aggressive behavior immediately and praise good behavior consistently. A pheromone product, such as the Adaptil collar or DAP plug in diffuser, may decrease any anxiety or aggression.

Walking them in neutral areas away from his normal routine and territory can help to get dogs used to each other in a less confrontational manner and can help in such cases, I would recommend muzzling both before allowing free play to prevent injury initially. Do not leave them unsupervised or loose until they are more accepting of each other

it is important to remember that Chico was there first, and is used to a certain level of attention from you. This newcomer has no expectations, so be sure to dote on Chico as much as ever so he feels secure. Then, gradually, you can increase the amount of time you give the new guy (without taking anything away from Chico). The second issue you are facing is that the dogs have to sort out, for themselves, who's who at the zoo. In other words, they may have one order of inportance to you, but they need to determine amongst themselves who's top dog, who's in the middle, and who's on the bottom. There may be a bit of snapping and barking going on, but as long as there are no injuries or serious fighting, leave them to it. Once they get it squared away, everyone will know his place in the pecking order and will (usually) behave accordingly.

There are some articles here that may be worth reading. http://www.veterinarypartner.com/Content.plx?P=SRC&S=1&SourceID=47 Every new dog is a new pack member with thier owne personality. A new dog is challenging your other dogs place and potentially taking resources (food, toys, attention). You need to slowly make introductions and not leave the dogs alone unsupervised until they have reached a stable peace. http://www.veterinarypartner.com/Content.plx?P=A&A=2465&S=1&SourceID=47

I'm sorry to hear that things aren't going well with your new dog. Unfortunately, it sounds like this may not be a good match - just like humans, some dogs have very different personalities and may simply not get along. If you want to try keeping both dogs, I would really recommend getting in touch with a veterinary behaviorist who can evaluate the dogs in person to give you a better idea of whether this is a fixable issue - if so, he/she can help you put together a detailed step-by-step plan to work on the problem. You can search for a behaviorist in your area here: www.dacvb.org In the meantime, I would recommend keeping your older dog on a leash or physically separated from the other dog with a baby gate or other barrier. This will allow the younger dog to relax a bit without being harassed. If things go well like this for a few days, you can gradually allow short periods of contact between them. Reward both dogs when they are together, and particularly praise and reward your older dog for calm behavior around the new dog. You may also want to try a DAP pheromone collar on both dogs, along with a natural calming supplement such as Zylkene or Composure for your older dog to see if this helps a bit.