r/Parasyte

Reiko/Ryoko mentions creating him as an experiment but as far as I can tell(I've only just finished reading the manga, haven't watched the anime yet) there was no explanation of how this was accomplished.

We see examples of parasytes having difficulties changing their "roles" in a body after their initial infection phase ends. Migi can only stay an arm, Jaw can only stay a jaw.

The best I can imagine is Reiko/Ryoko somehow located five "failed" parasytes, convinced them and their hosts to get together in one place, and then Frankenstein'd them together. That's a pretty big leap of faith for the five parasytes since if it didn't work out they'd all be done for(Imagine being Right Leg, killing your host to participate in becoming Goto, except it's a fuckup and you discover too late that the dumbass in charge of Torso can't figure out how to reroute circulation so you just die!)

Edit: Just remembered the "failed" parasyte that got stuck being a dog's head. Maybe she found five parasytes which had infected various animals and convinced them it's better to risk the coinflip than spend the rest of their lives as squirrels?