I rescued a rat a few days ago and I've noticed something odd about him. He curls his tail a lot. Even when I had him outside of his cage, free to roam the bathroom. I dont have any pictures of him, so yay paint program.
I've never seen a rat do this before, so I thought it was maybe he grew up in a small cage? I don't know what kind of cage he lived in originally.
Your fellow is a wheelrunner. He has what we call a wheelie tail. Obsessive wheelrunners carry their tail like this when they run on the wheel. You should probably get him one (make sure its big enough) since it obviously makes him very happy.