From the Super Mario Wiki
A Merfle Barrel is a barrel-shaped cage that the Shake King imprisoned the Merfles in during the events of Wario Land: Shake It! Wario must frantically shake the tiny prison to free the Merfle so he can show Wario the way back to the start of the stage on most levels, before the Shake King nabs and defeats Wario and takes all his coins. Most of the time, the barrels will be located at the end of the level. Alternatively, some levels like Launchpad Labyrinth and Lowdown Depths have the Merfle Barrels located at the start of the level instead.
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