Brown

Brown (ブラウン) is a robot in Mega Man 6 that mainly appears in Knight Man's stage. It resembles a barrel with a row of openings around the middle, and attacks by periodically firing an arcing projectile. The barrel is indestructible, but shooting it will cause the enemy to reveal its weak spot: the Met-like head contained within. The player must then shoot the head in order to defeat the enemy. A charged shot from Rush Power Adapter can destroy the barrel (and the Brown inside) in one hit without needing to expose Brown's head.

Octone, an enemy from Mega Man 9, behaves similarly.

Trivia

 * Brown may be inspired by a children's toy/game from Japan, Pop-up Pirate. In that game, players take turns poking plastic swords into a barrel, one of which may cause the aforementioned pirate's head to pop up and eliminate that player from the game.