Original: Two robotic versions of Knuckles have appeared (similar to Metal Sonic); Metal Knuckles in Sonic R, as an unlockable character, and Mecha Knuckles in Sonic Advance, as a boss. The robotic Knuckles in Sonic Advance looks exactly like Knuckles, only with a pinker hue and yellow gloves at first, and has virtually every move Knuckles has.
Simplified: He first appears in the game Sonic & Knuckles.