The Extra Life is a clone of Sonic the Hedgehog created by Doctor Kintobor before he became Robotnik. It only appeared in STC Issue 13 story Double Trouble.
It is unknown exactly how this 'extra life' was created, nor is it known why it turned out evil or how it became imprisoned. It was left abandoned in the Emerald Hill Zone for some time until it was found by Tails whilst playing football. Believing it to be Sonic, Tails activated the monitor device that contained it. The duplicate was released and went on a rampage across the Emerald Hill Zone, destroying the village and starting a forest fire.
The Extra Life went toe-to-toe with the real Sonic and very nearly defeated him, until Tails caught up to it and re-imprisoned it in the monitor. Sonic then kicked the box so hard that it flew into space. It has not been seen or heard from since.