"Go at 'em, Megaleg! Crush him with the power of your Grand Star!"
-Bowser Jr. coaxing Megaleg to destroy Mario.
Megaleg is a Snifit-like robot that Bowser Jr. sends out at Mario in Super Mario Galaxy. It can only be defeated by being climbed up on and then using a Bullet Bill to release the Grand Star. Its only attack is to try to stomp on Mario, which is simple to avoid considering how slow it is. After Mario begins to climb on it, alarms go off and it releases Bullet Bills, which Mario can use to free the Grand Star and destroy the robot .
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.
In the game, Super Mario Galaxy 2, a new version of Megaleg appears, named Digga-Legs but this time is defeated with the drill ability, is powered by an ordinary power star, and it keeps the star on its underside instead of the top of its head.