Metal Tails is robot created Dr. Eggman, similar to Metal Sonic only smarter, based on Miles "Tails" Prower, Sonic often confuses his friend to his Metal Entity. He appears as an unlockable character in Sonic Riders: Cloning age.
Metal Tails is created by the cute fox to protect the Master Emerald for Knuckles. At the end of the game, Metal Tails has been mind-controlled by Robotnik. Robotnik takes Metal Tails in his eggmobile. Sonic couldn't catch up and Robotnik got away.