Mystery Egg, also known as the Egg, is an enemy in 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 Mystery Egg itself has 0 HP, it will take on the HP value of whatever monster is inside. In the PSP version, the egg has whatever weakness/resistance the monster inside has. Likewise Steal will obtain the monsterās treasure.
Known monsters that can hatch from the Mystery Egg are: Black Lizard, Lamia, King Naga, Green Dragon, and Yellow Dragon.
In at least the Pixel Remaster version, the egg hatches only after it is attacked, including Osmose or Steal. A sufficiently strong attack can still kill it before the monster inside emerges.