My old dog won't get along with my new dog. What should I do?

I'm sorry to hear that you're having these problems! Aggression or fighting between dogs in the same household is a complex behavior issue that's very difficult to address without evaluating the dogs in person, because there are so many factors to consider - it's important to assess both dogs' personalities, how they interact with each other when they're not fighting, how severe the fights are when they happen, what seems to trigger the aggressive behavior, etc. For this reason, I would strongly recommend getting in touch with a veterinary behaviorist or a good reward-based trainer who can work with you in person to help determine why the aggression is occurring and put together a detailed treatment plan to address the problem. In the meantime, I would recommend keeping the younger dog on-leash whenever he is around the older dog to prevent any bullying or harassment, or using a baby gate or other barrier to keep the dogs separated until you are able to get some hands-on guidance in how to help them.

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.

It can be very stressful for the existing dog when a puppy is introduced. I am not sure what you have tried already but if it is not working then you could try an Adaptil collar, Solliquin chew, or talk to your vet about trying an anti-anxiety medication, like Trazodone to see if that helps relieve any anxiety your adult dog may have. Otherwise, you may need to get a professional trainer to come and work with your dogs. Here is a link with information on how to introduce a new puppy... https://indoorpet.osu.edu/dogs/new_additions_dogs/dog-dog-intro

You will likely need to separate the new dog from the 2 "older" dogs for a period of time. Spend time with each separately, but make sure that they cannot see the new dog, i.e. keep him in a separate part of the house behind a closed door for at least one to two weeks. Then you can allow them to see each other, but not have contact. Move slowly, from this phase on to allowing them to see and smell each other across a barrier such as a baby gate. Finally, allow them to spend supervised time together. Reward good behavior with high value food treats (i.e. bits of cooked chicken) and go slow.

Puppies can sometimes be A LOT for senior dogs and it can take some time for them to get use to one another. If there were any food, treats, toys or special people near by Buddy could have just been acting protective over these. Also, with Buddy being older, if they puppy is always playing, jumping on him, or nipping at him, he may have been just trying to tell the puppy to leave him alone or trying to set some boundaries. Also, if he is arthritic and he knows the puppy jumping on him hurts, he could have reacted to prevent this. I would continue to keep them separate until you know they can be left alone together. I would definitely feed them separately as well. I would be sure you are still spending plenty of alone time with Buddy as he may be feeling frustrated that his home life has changed with the new addition. I would continue to have short but frequent times of them being together. You could also start Buddy on an anti-anxiety chew like composure or composure pro and get him a calming collar like sentry or adaptil. If these don't help, talk to your vet about anti-anxiety medicine like fluoxetine or trazadone. I hope this helps!

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.

Pack dynamics aren't really a thing with dogs, (it's a construct of wolves kept in captivity) so it may be that they are still in the "settling in" period, especially since you didn't mention how long ago you added in the third dog. Adding in a new pet can be stressful for even the most socialized and well-behaved of dogs, and as they try to determine what the new dog is like it may lead to scuffles. If they become upset, agitated or try to get away from each other, separating them all for a while in a time out and letting them relax before reintroduction can help. Removing items of contention such as toys, bones, treats, bedding, etc can also help to reduce any fights over these items. Pheromone diffusers placed in the house can also help reduce tension between new pets. Dogs are also very good communicators, so if one of your dogs hackles or growls, it's a sign that it is time for a time out period. You shouldn't scold or reprimand your dog for this, as that can lead to behaviors where they don't give warning prior to a fight. Instead, calmly just remove the dogs to different locations and try again later. If the problems continue even after a few weeks of being together, or seem to ramp up in intensity even with changes, seeking out a trainer or behaviorist in your area that can help you in person is best.