Megaleg

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Megaleg is a Snifit-like robot that Bowser Jr. sends out at Mario. It can only be defeated by being climbed up on and then using a Bullet Bill to release the Grand Star. He is seen in Super Mario Galaxy.

According to an interview with the director, Megaleg was one of the first elements included in Super Mario Galaxy. The team appreciated the fight so much that they included it in the E3 2006 demo and eventually the final video game. It's unarguably one of, if not the biggest boss in the Mario series.