The Chomp Mushroom is a power up that is set to appear in Super Mario Galaxy 3. Once obtained, Mario becomes Chomp Mario. Chomp Mario can transform into a chomp and roll around, crushing enemies and other things things in his path, similar to Rock Mario. When Chomp Mario is a chomp, he can survive in lava, just like the golden chomp which comes out of lava in the first level of the Chompworks Galaxy in Mario Galaxy 2 Super Mario Galaxy 2.
A Chomp Mushroom (above) as it is shown in Super Mario Galaxy 3.