I think it is normal. Two of our dogs - the mixed breed and the male dachshund - both of them have hair on their backs that stand up when they get excited. On Carly it's just the hair between her shoulders, but on Max there is a strip all the way down the middle of his back. He's so short and he looks so silly when he gets all puffed up to protect us and the hair on his back stands up. I know he could be tough if he wanted, but all I can think of is "What's he gonna do - gnaw someone's ankles off?!?"