Super Knuckles

Super Knuckles is Knuckles' Super Form once all 7 Chaos Emeralds are collected. The 1st appearance of Super Knuckles was in Sonic 3 & Knuckles. Super Knuckles glows a flashing hot pink & unlike Super Sonic, which alters some appearances such as his running, Knuckles uses the same sprites as his normal form. Unlike his Hyper form, that can be obtained when the 7 Super Emeralds, Knuckles does not recieve any special powers, he is only invincible until he runs out of Rings.

That was the only appearance of Super Knuckles unless you counted Sonic Heroes, where he had a very poor super form, some fans were not happy about that & do not believe that it is a Super Form.