Monster-in-a-box

Monsters-in-a-box are certain enemies that appear in Treasure Chests. After the character opens the Chest and recieved the items, the word "Monster-in-a-box!" follow, and a battle begins with an enemy. Usually these enemies are stronger than average foes, and can even be bosses. Monsters-in-a-box were series staples up to Final Fantasy VII where they do not appear, and since have become less common.