Monster-in-a-box

Monsters-in-a-box are enemies that appear in Treasure Chests. After the character opens the Chest, 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 match a bosses 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. In the later games of the Final Fantasy, Mimics 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 of the game. 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. Astaroth is guarding a Ribbon. Beelzebub is guarding the Genji Armor. Zombie Borghen is guarding Genji Gloves. Tiamat is guarding a Genji Helm.

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 recieved. In the DS version the word "Monsters" has been replace by "Enemy Ambush".

Final Fantasy V
Monster-in-a-box appeared when the player opened certain Treasure Chests, and was commonly extremely strong for the location, but very rarely actually "guarded" something, as working 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 though, and only contain monsters.

Crisis Core -Final Fantasy VII-
At Gongaga, Zack can find treasure chests that contain enemies.

Final Fantasy IX
Many Mimics appear in Burmecia disguised as Treasure Chests.

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.

Final Fantasy XII
In Final Fantasy XII several urns may be in fact insectoid mimics. These can be found in the Barheim Passage, Lhusu Mines or Henne Mines. Also, there are three Crystals in the game that may be Crystal Bugs. One is located in the Stillshrine of Miriam and is disguised as a Save Crystal, one resides in the Sochen Cave Palace and is disguised as a "Life" Crystal and the last (and strongest) resides in the Nabreus Deadlands.

Final Fantasy XII: Revenant Wings
Like in Final Fantasy XII, urns in the field may be actually mimics in disguise.

Final Fantasy Tactics A2: Grimoire of the Rift
In Final Fantasy Tactics A2: Grimoire of the Rift, some treasure chests can contain mimics instead of treasure.