The answer? He's either from space, or a stardroid.
It may be a far-fetched conclusion to make, but let's take a moment to consider Maestro's abilities. For example, Maestro, albeit with a bit of assistance from Guts Man, can LIFT A FREAKING GUTS DOZER OVER HIS HEAD AND THROW IT DOWN SO HARD IT MAKES A HUGE CRATER. That means he has to be at least as physically capable as Guts Man, A robot, who mind you, Can lift 2 tons.
Now let's discuss Weapon Capacity. In the games, Mega Man could only hold up to 8 weapons, and 3 support units, as seen in Wily Tower. The most weapons Maestro can collect, to my knowledge, is during the fight with Gamma. If you've beaten Ballade in Knight Man's Stage, you'll get the Ballade Cracker, and you can get every weapon from MM1 to MM6, even including the ones from PU. In total, that's 50 WEAPONS, also including the buster you start with. Without cheating or through custom maps, that's his limit. Not only that,but he an have a crapton of support units. I don't know the specific amount, but I'd say MUCH more than 3.
Speaking of the fight with Gamma, Pre-V5B, Gamma could only be hurt by stuff like Atomic Fire, Ballade Cracker, Pharaoh Shot, and Hard Knuckle. In V5B, Wily explicitly says that nothing was supposed to hurt Gamma, Pretty much. However, in V5B, almost EVERY weapon can hurt Gamma. Perhaps that's similar to Terra oneshotting Mega Man? Or how about Mega Man not being able to do ANYTHING to Terra, pre-Mega Arm?
Speaking of THAT, let's talk about Durability. In the MM4 Chapter, Maestro is knocked down a giant hole into the trash compactor, which is stated that no ordinary robot could survive landing in, and is just fine immediately afterwards, even if he apparently loses consciousness for a brief time. The same happens with Gamma's fist pretty much leveling a building. (By the way, I would have LOVED to have a final battle against Impostor Mega Man.)