Mons Meg (Rebecca in the Japanese version) is a miniboss that appears in scene 4-1.
Depending on the version, the boss is either a robot (Mons Meg) or a giant doll (Rebecca).
Mons Meg has a castle atop his head and towers on his "ears", and several cannons as well. He has a variety of meters and aim symbols on his body as well as a lot of colors (red, yellow,green, blue, etc.).
In comparison, Rebecca looks a lot more simplex, but more in line with the game's puppet motif. She wears a green and yellow cute shirt with a green hat and a red ribbon in the back. She has long brown hair and a red ribbon on her shirt.
Before the battle with Mons Meg, the player must make sure to stop at the right moment because Headcase appears to give Headdy either a War Head, a Bomb Head or a Hammer Head. If the player keeps on walking towards the miniboss and don't stop for Headcase, there's no way to turn back once in the miniboss screen. Once the battle starts, the miniboss will try to smash Headdy using a big fist. Afterwards, a lock-on display will appear and the robot will try to aim at Headdy to shoot streams of bullets (music notes if going against Rebecca). The player must aim for the robot's face, his weakspot.