Doctors say the teenager might have developed the tail in the womb as a result of a spinal deformity, but that it appeared outside only after he grew up, as it started to appear on the 18-year-old's back just after his 14th birthday.
His family and him, from Nagpur in India, had kept it a secret because they were worried he would be bullied.
It's thought to be the longest ever recorded on a human.
"When the size of the tail grew it began to press on the boy's back," said surgeon, Dr Pramod Giri.
"It was cosmetically and psychologically disturbing for him", he told to BBC.