Ratty — the friendly squirrel with a peculiar tail

Can anyone tell me what might have happened to this visiting squirrel?
It looks a little like a rat until it skips off. It does not seem as agile as the other squirrels. Is this because a bushy tail gives better balance? But it seems to be surviving okay. If this is a defect I wonder if more squirrels will be born with it. Red Squirrels, grey squirrels and ratty-tailed squirrels!
We call the little fellow Ratty!