From the Super Mario Wiki
Micro Mecha-Bowsers (alternatively Micro Mecha Bowsers), initially known as Mecha-Bowsers, are a robotic version of Bowser. They loosely resemble Mechakoopas except their design is drastically different, they possess different attributes and, contrary to the name, are somewhat bigger and cannot be picked up by Mario or Luigi.
In Super Mario Galaxy, Mecha-Bowsers are found in Toy Time Galaxy and can breathe flames for fairly long amounts of time. They can even chase Mario, like Goombas do. They can only be defeated with a Ground Pound and stunned with a Star Spin. When defeated, they release a coin. They always tend to breathe fire right after recovering from being stunned.
In Super Mario Galaxy 2, they appear in the Flipsville Galaxy during the mission Flipsville's New Digs in the first section of the galaxy, and in the Cosmic Cove Galaxy during the mission Exploring the Cosmic Cavern on the last planet. In addition to the Ground Pound, Mario can also use the Spin Drill to defeat them by jumping over them and then drilling into them. Despite the new name Micro Mecha-Bowser, they are not any smaller than they were previously. The name is intended to be in relation to the giant Mecha-Bowser, which does not appear in this game.
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