Why is my older dog distant from me and aggressive with new puppy?

Unfortunately, this is a really common change in 2 year old dogs. Molly is reaching social maturity and things that used to bother her slightly but not cause a reaction are causing BIG reactions. She was probably always a bit nervous around other dogs, and now she's showing it. I recommend backing off her interaction with other dogs, so she's not practicing this aggressive behavior. No dog parks, etc. Then ask your veterinarian for a referral to a good trainer who can work with you on counter conditioning and desensitization to help Molly learn that other dogs aren't scary. This book is also very helpful if you don't have access to a trainer: https://www.amazon.com/Feisty-Fido-Help-Leash-Reactive-Dog/dp/1891767070

It sounds as if Coco may be a bit stressed out by the new puppy. Giving her some time on her own to relax and settle in with the changes is best. You can try spending some more one on one time together by offering a favorite treat, playing with a favorite toy, or just petting and grooming her without your other dogs present. Making sure each dog has some personal time as well as places they can rest such as their own bed, room, etc as well as making sure there are extra food and water bowls can help reduce stress and prevent tension between all three dogs.

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!

It may take some time for her to accept him. If she is being aggressive toward him than you should not allow them to be together unsupervised, espeically around feeding time or any toys/bones that she may protect. Give her extra attention, praise, and treats while the puppy is around so she knows he is not there to replace her. You could also try an Adaptil collar or diffuser and see if that helps. If she doesn't start accepting him than you could talk to your vet about trying an anti-anxiety medication, like Prozac or Trazodone and see if that helps.

It's very common for older dogs to not like a new puppy much at first - puppies can be rude, pushy, and overly playful for an adult dog :). You will need to give your older dog plenty of time to adjust to the pup at her own pace. Make sure she has someplace she can go to relax if she doesn't want to be bothered, and supervise them closely to make sure that the puppy isn't allowed to harass her when she doesn't want to play. It's also very normal for adult dogs to growl or snap at puppies who are being pushy and rude - this is how puppies learn manners, and your older dog should not be corrected for this. Things should improve over the next 2-3 weeks as they get used to each other. If there is no improvement in this time, or if you have concerns that your older dog may injure the puppy, then I would recommend making an appointment with a veterinary behaviorist for some hands-on guidance.

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 is possible that Coco just doesn't like the new puppy, but there could be other causes for her distancing herself from the rest of the family, including pain from arthritis, cognitive dysfunction, and internal medical illness, among others. I would recommend that Coco have a good checkup with a vet to rule out the easier possibilities. If there are no obvious causes found on examination and lab work, then the vet may recommend treating anxiety or supplements for cognitive dysfunction.