Megaleg is one of several bosses in Super Mario Galaxy. It is a gigantic mechanical construct with a vague resemblance to a Snifit. It is found in Bowser Jr.'s Robot Reactor, and is owned and possibly commanded by Bowser Jr.
Megaleg underwent several changes prior to the release of Super Mario Galaxy: in an early version of the Good Egg Galaxy, there was nothing in its cage. The earliest version of Megaleg was labeled "BossCrab", which had a different body design, no nose or mouth, four legs, and a half oval shaped head.
Megaleg is incredibly large and lives on a planet just big enough to hold it. It has three long legs, like a tripod, and each leg has various tricks and traps designed to trip Mario up. It has fly-like eyes with huge lights beaming out of them. On top of Megaleg's head is a red cage with a rotating lighthouse-like light on top that contains a Grand Star. Megaleg is much bigger than Digga-Leg, a similar boss encountered in Super Mario Galaxy 2.
When Mario enters Bowser Jr.'s Robot Reactor, Megaleg is positioned on the bottom of the Robot Reactor. When Mario uses the Launch Star on the Starting Planet to reach the Robot Reactor, Bowser Jr. appears and orders Megaleg to attack Mario. Megaleg mainly attacks by consistently firing multiple Bullet Bills from various locations on its head, legs, and underbelly. It also tries to stomp on Mario while he's on the Robot Reactor. Mario must jump on one of its three huge feet and climb up its leg, avoiding Bullet Bills along the way. When Mario reaches Megaleg's head, he has to lead a Bullet Bill into the larger cage surrounding the red cage, after which a barrier forms around the red cage and more Bullet Bills are fired. Mario must then lead another Bullet Bill into the red cage to release the Grand Star trapped inside and defeat Megaleg.
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A Grand Star is at stake in the battle against Megaleg, so Mario knows it won't be easy. With its flylike, light-beam eyes and three huge mechanical legs, this boss lives on its own planet in Bowser Jr.'s Robot Reactor. To defeat Megaleg, confuse the monster's own Bullet Bills so that they slam into its head and destroy it.
- The concept of climbing atop and moving on a large being was originally intended to be used in the final fight against Ganon in The Legend of Zelda: Ocarina of Time, but this was scrapped due to the fact that one would be unable to see Ganon while atop him. However, it was later reused in Super Mario Galaxy, much to the excitement of the development team.
- In a Nintendo Power interview, it was stated that Megaleg would return for Super Mario Galaxy 2. This was later proven false, however, with Digga-Leg appearing instead.
- There is a glitch whereby the player can access Megaleg's planet without triggering the cutscene or battle. Furthermore, if the player climbs to the top of Megaleg and enters first-person perspective, an unused green and brown planet can be seen.