I had a friend with a dog like this. I only caught glimpses of it when I went over. Whenever they had visitors they had to put the dog outside or in the cellar. He was a golden retreiver.

My uncles dog won't let anyone near it. I went over to him and had to back off. He was a pitbull, rottweiler mix. A big dog, all muscle. He wasn't like that as a puppy.

I really don't have an answer, but you're not alone. It must be because of they way they were treated.