From the Super Mario Wiki
The Golden Chomp in Super Mario Galaxy.
The Golden Chomp is a special Chomp in Super Mario Galaxy and its sequel, Super Mario Galaxy 2. It appears in the Gusty Garden Galaxy, where it plays a prominent role as the hidden star that can be found during the "Gusty Garden's Gravity Scramble" mission. It contains a Power Star inside, but Mario is unable to break it open without the use of a Rainbow Star, which can be obtained after collecting a series of ? Coins. This power-up allows the player to destroy the Chomp, revealing the Power Star.
In Super Mario Galaxy 2, it appears in the Chompworks Galaxy's first mission, "Where the Chomps are Made of Gold". Here, Mario must lead a Golden Chomp from the end of an area all the way back to the front on the area. However, it takes a set route and Mario must alter the area slightly to allow the Golden Chomp to get back to the front of the area and get the Golden Chomp into a hole, where it will break and reveal the Power Star. It reappears in the Chompworks Galaxy's Prankster Comet mission titled "Cosmic Clones in the Chompworks". Here, Mario is sent directly to the end area, and has to lead the Golden Chomp back to the beginning, but now Cosmic Clones are following him, and touching them causes Mario to take damage.