From the Super Mario Wiki
The Mystic Egg is an item found only in Paper Mario: The Thousand-Year Door. It is a pale white-and-gray egg found only in the Boggly Woods, where it is given to Mario by Petuni, Punio's little sister after playing the game "Stump Petuni" with the young Puni. It recovers five HP.
Names in other languages
||Bubu Egg ("Bubu" is the German term for Puni)