From the Super Mario Wiki
The Megamorph Belt is one of Bowser's weapons in Mario Party DS. Unlike the Minimizer, Bowser does not reveal it until close to the very end of the game, after Donkey Kong breaks the Minimizer. Bowser uses its power to transform into Blockhead Bowser, a form of blocks that Bowser controls. The weak spot of the Blockhead Bowser is the golden Bowser Block, which the player has to attack to defeat Bowser in Bowser's Block Party.
Although it is a belt, Bowser appears to wear the Megamorph Belt on his neck.