digibutter.nerr

[Spiny]

Also known as "Last Resort"
(The head is angled toward it's left, to get a good view of the eyes.)
My image of Metal Sonic during Sonic Heroes has him thinking ahead, even comtemplating his loss. He bio engineered a special mutagen, and created a carrier drone to release it upon his defeat or demise. When Super Sonic and his allies defeated the Metal Overlord, this unit activated, and scoured the planet in search of a host.
The mutagen will alter the physical structure of the host to vaguely match that of Metal Sonic. The drone itself has a fragment of it's master's memory, and uses the pincers to project it into the host's mind, thus re-encarnating the would be ruler. The catch is that it only works as long as the drone has a proper hold of the host. If it is forced to leave or destroyed, the victim will act as the once did, but temporarily keep their mutations, which will slowly fade away. Last Resort can move as fast as a formula one racecar, but that would break it's small engines after a matter of minutes. It's claws can shread through the plating used on a golden Egg Pawn with ease.
In short, it is small, but one slip up in dealing with it could cost you your mind and body.

[Medikoopa]

Cool! It'd be insane if there was a swarm of them, just to prevent complete erradication of all chances of re-embodiment.

Perspective seems off, though...

[Spiny]

The mutagen was too painstakingly difficult to make that much of to explain why there's only one of it story wise.
The actual reason is because I want it to be fair should the little bugger find a way into my alt slot.

[Medikoopa]

Hmm... Suggestion:

Unless if the body manufactures the mutagen, the head might be able to detach from the body.

Or perhaps the 'bug' could be able to burrow into the hide of its fleshy victim and entangle itself in its muscles, making extraction excruciatingly painful for the host.

[Spiny]

I forgot to mention the bug has to "hack in" by injecting the mutagen through the head, and positioning it's pincers just inside the skullcap.
Burrowing wouldn't be the best choice from that angle.

[Medikoopa]

So... It grips the back of the head with its feet, sinks its 'nose' into the victim's skull, and ensures the undisturbed injections by digging its 'teeth' into the outer-layer of skull.

Cool!