Mecha Knuckles was a robotic-double of Knuckles the Echidna who acted as a minor villain and boss fight in Sonic Advance, beginning as a rather convincing replica of Knuckles (save for pink coloration) Mecha Knuckles soon revealed his true form as a more primitive machine in a style not unlike Silver Sonic.
Mecha Knuckles, a robotic version of Knuckles the Echidna, made an appearance as a boss in Sonic Advance. He had virtually every move Knuckles had, plus the ability to fire rockets from his mouth in his second form. He looks like a magenta Knuckles with yellow gloves at first, but after being struck four times his "armor" will fall off, revealing a metallic echidna with glowing red eyes underneath. After being attacked another four times, the robot explodes. The current player character seems slightly stunned when seeing Mecha Knuckles for the first time, but Knuckles' reaction is comically over-the-top when he sees his robotic doppleganger. He appears in the Angel Island Zone of Sonic Advance.
In the Archie Sonic the Hedgehog comics, the Freedom Fighters were once forced to roboticize Knuckles to combat a roboticized version of Sonic, called "Mecha Sonic" and the robotic Knuckles was dubbed "Mecha Knuckles". After a drawn out battle, both combatants were eventually restored to normal.
In the British Sonic the Comic, he was a new version of the Metallixes, loyal only to Doctor Ivo Robotnik and with increased strength. It made its debut capturing Sonic for the Drakon EmpireSonic the Comic #108, "The Evil Empire, Part 1" and then acting as a Prosecutor in Sonic's trial-by-combat, and it came very close to killing him until Tails destroyed it by sheer luck. The Metallix was only reused once, as Grimer used it to defeat Sonic so he could discover from him what happened to Robotnik; he then faked things so Sonic wouldn't remember this and instead think he beat Metallix. In its three clashes with Sonic, the Knuckles Metallix has never been defeated. It was shown at one point that Robotnik had a whole army of them, but they were never shown in use and may well have been eliminated when Sonic cleared out Flickies' Island.