Artwork from Sonic Boom
Sonic the Hedgehog
Metal Sonic is the evil robotic counterpart of Sonic the Hedgehog, created by Dr. Ivo "Eggman" Robotnik. As well as resembling Sonic, he can also perform many of Sonic's moves, including his homing attack. Metal Sonic made his first appearance in the Sega CD game Sonic CD. Metal Sonic kidnapped Amy Rose, who also first appeared in the game, in whom which Sonic had to save her. Metal Sonic is always on the dark side throughout the series, with Sonic Rivals 2 being the only game where he takes a protagonistic role as a playable character. Metal Sonic also made an appearance and was the true villain in Sonic Heroes.
In M.U.G.E.N, Metal Sonic has been made by several authors, but the most notable ones are SuperMystery's version and MugenHunter's version.
Surprisingly enough, SuperMystery's version plays more like a playable Sonic character than a fighting game character, or even a boss-fight. As such, Metal Sonic has similar gameplay to SuperMystery's Sonic, Tails, Knuckles, Espio, and Vector, utilizing many of the same power-up monitors as well.
MugenHunter's version, also known as Metallix, is the boss character of MugenHunter's first Freedom Fighters. He carries moves based on all of the playable characters, as well as a few of his own moves thrown into the mix, such as his laser beams and electric shield.
Another note-worthy Metal Sonic M.U.G.E.N character was made by Txpot. While being of decent quality, Txpot's version is rather unconventional in terms of his moves.
Plom5-1-00 is mostly known for Bingo the Chao, who was nominated for "Worst Mugen Character" one year at Random Select. As such, Plom5-1-00's Metal Sonic is generally of poor quality, only harboring normal punches and a one button missile projectile.
Shazzo's version, like many other Shazzo M.U.G.E.N characters, is a spriteswap of Actarus' Axel Stone.