Mystery 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.
The Mystery Egg has a set HP of 10,000 and it absorbs all elements and is immune to all status effects.
Known monsters that can hatch from the Mystery Egg are: Black Lizard, Lamia, Nagaraja Green Dragon, and Yellow Dragon.