This thory is going off of Byuku-Sharingan thoery of the 10th bijuu. There were originally 10 akatsuki members and ten rings, and when orochimaru left he took his with him, that is why we only have 9 members now. But why 10 rings and ten members. Orochimaru left with his ring so we only have 9 members now. But what if orochimaru stayed, there still only 9 bijuus. One for each member right, so i am guesing there is a tenth bijuu that shows up after all the other nine have been collected, or there could be one that we dont know about. Now i know you guys will say there are only 9 in mythology, but this is not mythology, is is a story created, so the 10th bijuu can just as easily be created.