After our beloved dog Xesca passed away, we decided we were ready to welcome another dog into our home. When we first met Sam he was malnourished and very scared, so we went to the shelter for three days and walked him and gave him treats so he would get to know us a little bit before bringing him home. Once at home, he wasn't scared anymore and he turned out to be a very sweet dog. We've had him for almost six months now and his temperament has changed a lot since we got him. He only responds to my father and has stopped responding to me or my brother. He's very sweet until you try to make him do something that he does not want to do. If we ask or tell him to leave the room for whatever reason, he completely ignores us. If we approach him he throws himself on the ground so we can't pick him up (he's a very heavy dog) and if we try to pick him up or grab his collar he tries to bite (he already bit me last week). If we get him to leave he instantly goes back into that room again. The same thing happens in other situations. When I bring him to the barn, he stands or sits under different horses and I try to make him leave as it's very dangerous and he can get kicked. He won't listen to me and gets aggressive and dominant if I try to move him. If I manage to make him leave, he instantly goes back under that horse again. He has tried to bite my brother too and has started to growl and throw himself at strangers during walks. He's only dominant with my brother and me, he's very submissive with my dad and only responds to him. Our vet has checked him and no physical issues were found. I would appreciate any advice as I'm not sure what to do to better the situation. He's and 8-year-old castrated male. Thank you!!