| I think there are probably some other reasons that this owner is trying to rehome him. If he were truly this great, why isn't his breeder in the picture helping to rehome him?
Do you know his breeding and what sort of lines he comes from?
I think there are a few issues -
first, as Lexus said - male dobermans really do not belong at dog parks (I don't think most dogs do, period - but I would never bring an adult male doberman when other dogs are present - i use the dog park when it's empty and relish the 3 acres he can run loose on, or when dogs he knows are there). Your male doberman will likely already have, or will develop, some degree of male dog aggression.
Second, part of me is sad to say this, but I think part of what you are seeing is that "the honeymoon is over" sort of phenomenon, and his true personality is coming out. He was just a visitor at first, and now that he thinks he is staying, he is laying stake to what he thinks he should be.
Third - do you know if this dog has bitten before? If he guards something like a toy, how will he act around food, other high prize items like bones, or if he is on the couch later on and doesn't want to get off?
Fourth - yes, your relationship with your existing dog will change, it is the nature of having 2 dogs. If you are willing to let that change in this manner, that is a decision you have to make - each dog makes the relationship change ina d ifferent manner. I have three dogs (one newly introduced) - the first dog was barely affected by the second dog addition because that dog had minimal exercise and training requirements and was quiet in the house - if anything his life is only added by the second dog. The third, however, is intensive, and it DOES change how much time I can spend with the first dog, and he does feel that. He is no longer the only one to consider all the time.
Your first post does seem like you have some doubt as to if he is the right dog for your family, even before he bit you. If you are having doubts, I would return him now, before it 1) gets harder 2) the original owner disappears or 3) god forbid he hurts someone else, or bites you harder. If you plan on keeping him, I would get to work ASAP on obedience and work on his behavior with a qualified specialist.
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