Mystery Egg, also known as the Egg, is an enemy from the game Final Fantasy IV. It contains a monster within, and if enough time expires during the battle, the egg will hatch, revealing a random monster inside. Destroying the Mystery Egg before this happens will prevent hatching, but no EXP or gil will be awarded.
While the Egg itself has 0 HP, it will take on the HP value of whatever monster is inside. In the Nintendo DS version, the Egg has a set HP of 10,000. In addition, it absorbs all elements and is immune to all status effects (In the PSP version, the egg has whatever weakness/resistance the monster inside has).