How to introduce a rescue dog with guarding issues to another dog?

It sounds like Bruno has not been socialized well with other dogs, which does tend to make it difficult to introduce him to other dogs. Try introducing the dogs in neutral territory - like a park. If it's not their house, then they will be much less likely to fight. While having the muzzle on does add security, it can make the dog act aggressive. Same can happen if you are holding the dog or putting tension on the leash. The best and safest way to introduce them would be at a neutral place with a fence inbetween them. Let Bruno off lead to explore the other dog. If he's aggressive right away, then it won't work. If he's not aggressive and seems interested, then the next step would be to introduce them while walking on lead or in an open area off-lead. This is a bit more risky, so just make sure you're nearby to break up a fight, but don't be overbearing (i.e. holding the lead tight). I hope that helps and all goes well.

Start with a door between them so they can sniff each other. From there keep one in a kennel so they can see and sniff each other and you can gauge how they interact. If there is aggression then keep them separate still. If they seem to get along then pick a single cat to leave with the dog one on one while the dog is in a leash sobyiu can control the situation. From there you can slowly introduce the other the same way before placing them all together

You're right that there can sometimes be problems when introducing two dogs for the first time, especially if either of them has any history of aggression towards other dogs. I would recommend introducing them in a neutral place at first, like a park. Keep both dogs on leash, let them sniff each other briefly, then go for a walk together. This allows the dogs to get used to each other's presence without feeling pressured to interact. After the walk, take them both home and into the house together and if things are going well, let them off-leash to relax. If things don't go smoothly, it would be best to get help from a veterinary behaviorist (ideally) or a good reward-based trainer who can work with you in person. You can search for a behaviorist in your area here: www.dacvb.org or a qualified trainer here: www.ccpdt.org.

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.

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.

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.

The best way is gradually. Allow them to hear/smell but not see each other initially. This can be done by keeping them in separate rooms with a closed door between them. Once they are acclimated, you can have them see each other but do not allow physical contact. After they accept that situation, place one in a secure carrier or crate and place them in the same room. Eventually, physical contact can be attempted. Use leashes and muzzles initially if necessary. A pheromone product such as the DAP Diffuser or Adaptil collar may decrease any aggression and anxiety.