Monster-in-a-box

Monsters-in-a-box are enemies that appear in Treasure Chests. After the character opens the Chest, the phrase "Monster-in-a-box!", "Monsters!", or similar is displayed, and a battle begins with an enemy. Usually these enemies are stronger than average foes, and can even match a boss's strength. Monsters-in-a-box were series staples up to Final Fantasy VIII where they do not appear, and only return to the series in Final Fantasy X.

Previous to the use of monster-in-a-box treasure chest starting in Final Fantasy II, the original Final Fantasy surrounded treasure chests with fixed encounters to protect boxes. As a precursor to the monster-in-a-box, enemies fought through this method still had the tendency to be stronger than average. In the later games of the Final Fantasy series, Mimic enemies act as a replacement which appear as boxes and morph into an enemy.

Final Fantasy II
In Final Fantasy II, Monsters-in-a-box are very common, appearing in almost every dungeon. Unlike later installments, the item is gained before the monster is fought. All types of dragons appear as Monsters-in-a-box in some later dungeons and in the last dungeon, Pandaemonium, several of the game's most powerful armor pieces can be found in boss fights with Monsters-in-a-box.

Final Fantasy III
During the final dungeon of the game there is a crossroad of 4 path's each path has a chest along the way, filled with a monster-in-a-box: Xande's Clone guarding a Ribbon,

Final Fantasy IV
Many monsters-in-boxes can be fought in Final Fantasy IV. After opening a box, a message saying "Monsters!" will appear and the fight will initiate. After winning the battle, the item contained in the chest will be received.

In the DS version, the word "Monsters!" has been replaced by "Enemy Ambush." If there is an item, the item will been received, followed by the term "Enemy Ambush." For example, in the Giant of Babil, the message box will say  Found Elixir. Enemy Ambush!

Final Fantasy V
Monster-in-a-box appears when the player opens certain Treasure Chests, and is commonly extremely strong for the location, but very rarely actually "guards" something. Monster-in-a-box works more as a trap.

Final Fantasy VI
Several monsters-in-a-box appear in Final Fantasy VI. A notable example is Angler Whelk in Darill's Tomb, which drops a strong Holy-elemental weapon for Sabin.

Final Fantasy VII
Monsters-in-a-box only appear once in the clock puzzle room of the Temple of the Ancients. They do not give items.

Crisis Core -Final Fantasy VII-
In Gongaga, Zack can find treasure chests that contain enemies. During a mission he will also encounter a treasure chest that will transport him into a battle against Bahamut Fury.

Final Fantasy X
In the Omega Ruins, there are areas where three to five treasure chests are grouped together. Some contain treasure, while others initiate a battle. After the battle, the other nearby treasure chests will disappear. It is randomly determined which treasure chests are either a monster-in-a-box or a normal treasure chest in each playthrough.

Final Fantasy X-2
There are several monster-in-a-box chests in the Fiend Colony. The battles that initiate are against Vermin-type enemies.