Once their live a beast called Ten tails it had Jinchuriki who had 2 sons that fought over the ower of the beast. He could not take it anymore so spilt the beast into 9 beast that combined would create Ten tails. But it back fire he made ten other beasts the tenth one was Eleven tails but he was weak since he didn't have the strength like the others, he was die. But Nine tail took pity on him and slice off one of his tails and gave it to Eleven tails to help him live. This is how Eleven tails came by to get its eleventh tail and become brother with Nine tails along with how one of Nine tails has one short tail.