Thanks for the additional information. It is helpful. There are medical conditions that can cause sudden aggression in dogs. One is hypothyroidism and boxers are prone to this. Another cause is pain and boxers are also prone to cervical instability leading to disc issues in the neck as well. These can be painful as well. So before jumping to strictly behavioral in nature, I do think you need to rule out medical causes first.
If medical causes are ruled out, then you need to make some changes. Dogs are aggressive toward other dogs for a variety of reasons. It might be that they are fearful of other dogs and thus are aggressive before the other dog can be. In other cases, a dog is aggressive in order to dominate the other dogs and be the alpha member of the pack. Other causes could be that the dog feels they are the alpha member of the pack and as the alpha member they must protect the pack (you) from threats (other dogs). I suspect that your dog is a dominant female. Watch how she approaches other dogs. Is her tail or stub raised high in the air? Is she putting her head over the other dogs neck? These are dominant gestures and these alone can trigger a fight if the other dog is dominant.
You can look at the following pages on dog body language and it might help deterimine if it is dominance or fear based.
In addition, owners sometimes make the situation even worse by tensing up and worrying about what will happen. The dog senses the owner worry and feels that she is justified in her aggressive stance because you are obviously worried about the dog. They don't know you are worried about them attacking, they just feel that you are worried and assume it is the other dog. Even experienced trainers sometime worry and the dog acts up.
For a dog like this, you must be the boss. To accomplish this, you may want to have the dog wear a basket muzzle anytime she is not in your own house or yard. Not sure if you are using a basket style muzzle or not, but it is designed so the dog can eat, drink and breath normally. This will not only prevent bites but also allow you to feel more at ease when walking her. If she is not spayed, have that done.
Many dominant dogs are described as well behaved until you try to get them to do something they do not want to do, and then they reprimand you either with a growl or bite if you don't heed the growl. Things like taking away something they want, making them move when they don't want to, waking them up, etc can cause them to reprimand (bite) you. I'm glad she isn't at this point.
Dogs that are allowed on furniture tend to feel that since they are elevated to your level, they mentally feel elevated as well in the pack order and thus are the boss. Keeping them on the floor can help lower them mentally back to a submissive position in the pack. So the first thing is to not allow her higher that the humans or even on the same level. Attach a leash and use it to remove her from the furniture. Give a correction in the form of a quick tug and firm "NO" when she attempts to get on and a treat when she starts not trying to get on the furniture. Thus you are providing negative reinforcement for the getting on the furniture and positive reinforcement for the desired behavior (not attempting to get on the furniture). Hot dog slices or liver slivers work best as treats.
You will need to have her put back in classes. If you can, I would do group classes (with the muzzle if necessary) and let the trainer know of the problem your dog has. Before you can get into classes, the following site is helpful. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Obedience training serves various purposes. It helps a dog learn what humans expect of them when they state a command which leads to self confidence and less fear. Each time a dog obeys a command, even if it is for a treat, it makes them a little more submissive to that human in the future which helps with dominance aggression. It also socializes them around a variety of dogs and people. And since it is the leader or boss who is responsible for protecting the pack, if the dog is made submissive with training, you are responsible for protecting her, so that can reduce aggression due to fear and dominance.
You will also want to keep a leash on her at all times initially to grab if she should disobey. I think you will be pleasantly surprised at how well your dog does with daily training. Dogs like knowing what is expected of them and they love the little paper thin slices of hotdogs that I use for treats while training. Give this a try and see how it works for you. Use the leash to reprimand her for any attacks on the puppy. Growls are fine and even small lunges but she should not connect with the pupp. I would not even allow lunges if possible.
Since the pup is a male, you are liable to have more issues when he matures into a sexually mature dog around 7 months and then even more issues when he is a bit older and matures into an adult. Unless he is a submissive dog, he is likely to test the leadership of the boxer and then you are going to have a fight . So discourage all but mild growls from the adult female. In turn, you have to ensure the pup does not overstep his boundaries and keep him from taking things from her, trying to eat her food or jump her. Those are typical situations where an adult female will growl at a pup to show them it is unacceptable behavior. If she isn't allowed to reprimand him because she is too rough, you ave to reprimand unacceptable behavior. So start training him now as well even if you need to do it at home until he has all his shots.
It will be helpful if you can find someone with an adult dog to help you once you have your dog listening to commands consistently. What you will do is have your dog on the leash. You will have your helper off in the distance. Your helper will gradually move their dog a bit closer to you preferably walking past your position in the distance. As long as your dog ignores them, you can give your dog praise and a treat. The second you see her fixate on the other dog or show any other sign of aggression (hair standing up, etc.) give your dog a correction by giving a short tug and a firm low toned "NO". It shouldn't take your dog long to realize you will not tolerate the aggression and that if she ignores the other dog, she gets treats. Once this happens you can repeat the training moving the other dog closer until she is no longer trying to lunge at other dogs. You will need to practice this when you and your dog are walking as well.
In addition, if the situation is not improving using the techniques on the previous website, you may have to consult a professional behaviorist. You can usually find a behaviorist by asking your Vet for a recommendation or you may be able to find one using the following site.
I hope this information is helpful to you and you are satisfied with my response. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have .