Neo Metal Sonic
From Sonic Retro
Neo Metal Sonic is a transformation of Metal Sonic, featuring as the robot's primary form in Sonic Heroes. With elongated spines somewhat like Rocket Metal's, Neo Metal Sonic also sports spiky shoulder epaulettes and a billowing, feet-length cape.
Neo Metal is "mystery monster" of Sonic Heroes. Metal Sonic takes this form when he reneges against his master, Doctor Robotnik, and (via Froggy) acquires Chaos' ability to shapeshift. The robot imprisons his creator and takes over the Egg Fleet by impersonating Eggman, intending to conquer the world and create a robotic empire. To this end he lures in the various Teams of heroes, copying their combat data while disguised as Robotnik.
In the Last Story, Neo Metal Sonic reveals himself atop the spire of the Final Fortress, but only seconds later transforms into the dragon-like Metal Overlord. At the very end of the game he transforms back down to the classical Metal Sonic model, losing his lengthy spines and cape.
The name "Neo Metal Sonic" is never used in-game during the English version of Heroes, but it is confirmed in Sonic Rivals, where 'Neo Metal Sonic' is the 150th (and final) collectable card.