Here is an exciting (But unfortunate) photo of the Jazwares Mini Morphs prototypes. (These were a 2013 item) Why is it unfortunate? Because if you look at the MM's who were actually made, Knuckles is not on that list. But why not? He looks really good here, he's on model, and he fits right in with the rest of them...so why didn't he get made? It's a mystery that's just too bad.
Knuckles isn't the only thing that's unusual here. The other figures aren't exactly the same as the ones JW did finally end up releasing.

"Sonic takes stuff from the 5" figure, Tails takes stuff from the 3" figure, and Metal Sonic is metallic."