It started to appear on the 18-year-old's back just after his 14th birthday.
He and his family, from Nagpur in India, had kept it a secret because they were worried he would be bullied.
They finally went to see a doctor after it grew too long to hide - and had begun to develop a bone inside.
It's thought to be the longest ever recorded on a human - although cases are very rare.
"It became a problem when the tail grew outside the body," said his mum, who doesn't want to be named.
"He would just lift the tail every time he needed to change his clothes.