Billy has a marking problem when we go to my parent's too. For Bill, I don't think its dog specific; he just likes to be the boss.

Is Little Bit spayed? Might be a male/female/dominance thing.

This is a bit different, but when we adopted Walter, he wasn't fixed. My parent's dog, Morty, HATED him. I can't even express to you how much he hated him. Walter couldn't even walk by Morty.

After Walter was fixed, it was so weird. It was like a switch was flipped for Morty. They sleep together and play together now.