Mutagen is everywhere ... he's also been a massive alien who happens to look like the Teenage Mutant Ninja Turtles. IDW Publishing has introduced plenty of factions in their Teenage Mutant ...

and they accidentally released the canister of the mutagen ooze that gave life to the Ninja Turtles. They didn't want to take over the planet, and they certainly didn't want to harm the Turtles ...