this is Fire Sonic a transformation obtained from the "FireFlower" (a power up form Mario games), it appeared in Super Mario Bros Z (a YouTube series). although it's not official this form is one of my favorites
Actually, there is a canon (to the archie comics at least) transformation Sonic achieved called "Solar Sonic" He only had it for a few seconds, and technically it was only an effect of being Ultra Sonic, but it was cool nonetheless.
Things are about to get hot in here. Seriously, I like the look that you gave Fire Sonic here. I still want Sega and Nintendo to make a legit crossover (I don't fully count the Olympics games) where the two use some of the other's powers. Sonic gets flowers, feathers, and mushrooms. Mario gets Whisps. How cool would that be? XD