It could be any number of things this sounds like something your vet should inspect and offer some basic guidance. Two of my boys Frank and Dexter have small callous like spots on their tails that the vet's best estimate is that they are a results of the boys sitting on their tails much of the time. They both sit with a tucked tail a large part of the time. The rest of their til the fine it;s just one area that appears hairless and calloused.
Tom and Tanner, Gretel, Polly and angels (Aggie, Frank and Dexter)
"He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion"