A palette swap of Sonic, Ashura is accessible in Sonic 2 by hacking into the game with a specific code. Ashura has not appeared any time after Sonic the Hedgehog 2, so it is believed that he was just a graphical error or mistake. Either that, or Sega just decided to drop the idea. The hack was given the name "Ashura". Though it is said to be made by a fan, nobody knows who made up the name, so it comes from unknown roots. Since he bears similarities in color to Archie Sonic comic's Scourge the Hedgehog, some fans believe him to be a playable version of Scourge the Hedgehog. Others believe him to be Shadow, even though the idea of Shadow didn't come until years later. It is most likely just a graphical error or mistake due to the game being rushed.
Sega never revealed why Ashura appeared as the result of a hack, whether it be due to a graphical error or if it was meant to be a secret unlockable color swap, hackers first discovered Ashura in Sonic the Hedgehog 2. After performing the hack sequence, you have to press a button repeatedly in Emerald Hill Zone (while in debug mode), where many items from the game appear (item box, etc.) and keep tapping it until a green and black hedgehog appears, Ashura.