Tatepakkan

Tatepakkan (タテパッカン) is a shielded cannon enemy from the Mega Man series. They can only be hit when they raise their shield to fire, but if Mega Man can somehow get behind one, he can bypass the shield and destroy it more easily.

Mega Man 5
Tatepakkan appears in Gyro Man's Stage, Stone Man's Stage, the second and third stage from Proto Man's Castle, and the first Wily Castle stage. They also appear in three colors, but besides that there is no difference between them. They are weak to Super Arrow. The Charge Kick is also a good weapon to use on it as it can go straight through its defense and destroy it with two hits, usually hitting them twice with one attack. It is possible to hit through their shields at point-blank range.

Mega Man IV
Tatepakkan appears in Stone Man's Stage and in the second stage from the Wily Battleship. The shield hitbox is considerably smaller, so the Charge Shot and several weapons can damage it from below even when the shield is down. Beat can easily bypass its shield. Additionally, they can be stood upon without taking contact damage, which is required to obtain the Plate in Stone Man's stage.

Mega Man (Game Gear)
Tatepakkan, named Shield Gun in the game's manual, appears in Stone Man's Stage.

Hits Data Chart
Amount of shots/hits from Special Weapons it takes to destroy a Tatepakkan.

Other media
Tatepakkan has breaf appearances in the Rockman 5 manga.