From the very first outline, Sonic was going to undergo some sort of radical, terrifying transformation. Here are some concepts with that idea!
These I drew right as I was starting the animatic for part 1. See, there was the giant final boss monster from whom Sonic was to obtain his new power, and that needed to appear in the second shot of the whole thing. I had to figure out what it would look like pretty soon!
The idea with the monster is that it'd be representative of or even inflicted with the same transformation Sonic would later undergo, so designing it and Amalgam Sonic was something I sort of did at the same time. I had to figure out a look that would match both of them. By the way, you did notice that when the monster is defeated in part 1, it slithers off as a normal snake, right?
The whole thing with Sonic's cloning I wanted to emphasize is that it was taking snapshots of his own movements in order to form doubles of him, and so the Amalgam's design was somewhat inspired by those 3D zoetrope sculptures that look like they're animated when you spin them around really fast. Oh, and somewhat related is why the clones had a more hastily drawn style to them: the idea was that they were like in-between frames that had been given a life of their own. I honestly would have loved to go even further with that idea, but I only had so much time to work with. Maybe in a sequel?
The idea of Sonic's legs turning into a propeller that allowed him to hover above the ground was pretty fun, and directly inspired by my Sonic's Peel Out Problem animation. That one is almost something of a precursor to this idea, a weird body horror concept based on the cartoony stylization of certain animation tricks.
As for the monster itself, I'd cite my direct inspirations for that being Perfect Chaos and Dark Gaia specifically. I'm pretty proud of how it turned out, I think the inspiration shines through enough for Sonic fans to quickly understand, yet it also looks distinct enough to be its own thing.
The reason for choosing a snake has a few things too it. There's the imagery of a bunch of them intertwining and twisting around each other to create a whole new body that I thought seemed pretty natural. There's also the ouroboros, that iconic emblem of a snake eating its own tail to represent wholeness or infinity. However, this one isn't quite eating itself, but rather, two heads are perpetually butting into one another, as if the sum of its parts aren't coming together so evenly. There's also just the fact that serpents are common in the sort of Maya-inspired architecture I had in mind for its temple, although I wasn't really considering the symbolic significance to Mesoamerican culture in this case, so maybe don't look too hard into that side of it.
I suppose that's all there is to say on that, then! I hope you enjoyed seeing Sonic transform into a hideous creature, and I think I'll crap my pants if someone actually turns it into a Friday Night Funkin' mod. Seeya!