Hello everyone! I have grown up with male dobermans, and i have never seen one do this.
I rescued a 6 month old intact male doberman in July. He had no training whatsoever- he would not come, sit, down, nothing. He was kept inside all day with no one around when he was with the other owner, and was very socially backwards.
When I got him, i took him on walks daily to get him used to being around people. At first he was skittish, but then over time he loved being around people and other dogs.
About 6 weeks ago, we got another puppy (a bloodhound). And they are always playing together and the dog is very gentle with the pup.
Fast forward, he is now 10 months old and *for the most part* is very obedient. About a month and a half ago, my parents came from out of town (the dog had met them both many many times before this encounter) and my mom sat down, the dog viciously went at her. That was the first time that had happened. Now he won't let people in the house, and is becoming very aggressive. Last night i was just petting him, and he reached around and aggressively bit my hand (bruised and swollen).
He got fixed two weeks ago - and the vet said he looked and sounded healthy.
What can i do to stop this bad behavior?
It doesn't sound like you rescued, it sounds like you purchased this dog from someone that is rehoming. It's illegal, through PACFA for a rescue to adopt a dog out that is intact, unless they're breaking the rules. For those of us that do true rescue work this can be very insulting. Much more goes into rescuing than just taking a dog from a home that no longer wants it.
So you bought a puppy and then purchased another puppy? That's a bad idea, please look into littermate syndrome. Hopefully the bloodhound is a female, if not, also look into same sex aggression which is very common in dobes.
I don't like your words, they could be true (we weren't there so we can only go by what you've written) but it's unlikely and I'll explain why. You said your dog "viciously went at her". If he viciously went at her (and she was sitting down) she would be in the hospital. Just because your dog meets someone over and over doesn't mean that they trust them. I always tell my friends to NEVER walk into my house without being welcomed. Just the other day a fellow IGP member tried to be funny and surprise me at my car window. It just so happened Mav was in the front seat. Mav reached over me to get to him. Mav likes the guy, even trains with him, but not enough to trust him doing something that Mav perceived as aggressive towards me. Then you said he, "aggressively bit you". Once again, if he aggressively bit you you wouldn't be swollen and bruised. This makes me think he isn't feeling good and some pain came on him while you were petting him. I would highly suggest that you look into reading some books on dog or animal behavior.
Why would you then get him fixed? If a dog is in a fear period and you take away the hormones that help them mature and develop how will they ever mature out of that fear period? Keeping him intact and working with a trainer would have been the surest way to get him out of it.
Dogs go through fear periods until they hit about 3 years old. At a year old my male turned into a fear induced Kudjo and I had to work diligently to get him to not react to dogs and sometimes people.
IMO you've done many things wrong here to not set this dog up for success. When getting a puppy from a bad environment the best thing to do would have been to seek out a trainer then but let's move forward to what you can do now. I would take him to the vet and get a full blood panel done and maybe have the incision from neutering looked at and make sure nothing is still bothering him. He seems a bit young for thyroid or brain tumors but that could be checked out as well. Then I would seek a veterinary behaviorist so that they can see his behaviors and decide what steps need to be done to progress. Sometimes dogs are just wired incorrectly and the only solution is to be put down (that could be the case here). Without us being there, there's no way we can give you exact advice as to what to do.