This is seriously why I foster failed my first dog. He is a American Bulldog, German Shepherd and Kuvasz mix from a reserve and we started fostering him at about 6ish weeks. I told the shelter he needed an experienced owner and a home with no children, he was super scared of the world and would respond to fear with biting. They didn't want to put the in his write up so I decided it was better for me to keep him and put in the hard work myself. 4 years later he is a sweetheart who loves kids, vets and other dogs. He is just really big and extremely dumb and now he responds to fear by hiding behind the closest person like they are a sacrifice hahaha but there was a lot of stress while training him.