Consider that the coccyx in all great apes and most old world monkeys is a vestigial remnant from a time when all monkeys had tails.
With that, compare the coccyx of a chimpanzee with the tail of a new world Capuchin Monkey. Subsequently, consider the tail of an ordinary house cat compared with a Manx cat's tail.
Furthermore, the oldest ancestor species to all modern monkeys had tails. It is mostly unique among old world primates like ourselves to be without tails. Wouldn't this actually mean when humans grow tails, they would grow to match our older relatives:
