Is it ever stated who the Humungus was?

Humungous was.... a really humungous dude.

I'd say either Road Warrior Animal or Road Warrior Hawk

He was originally meant to be Goose, but they ended up not going with that. There are a few hints indicating their original plan, like the police revolver and burned/scared face. But he is just a bad guy with no background as to who he is.

Apparently it's true that early on the idea was the bad guy would wear a mask and there would be a big dramatic reveal of him being Goose toward the end. But that idea was THANKFULLY jettisoned very early on and nothing connected to it made it into the movie.

It's not a police revolver, it's military. And there's no reason to assume it was originally his or a family heirloom or anything. He's a post apocalyptic scavenger, odds are he found it in the ruins of a house or a pawn shop or something.