??? is what treasure chests are called in battle. There are two types of these treasure chests: one which the player can steal items from, and another one which stealing from will reveal its true form as one of four types of Mimics.
The treasure chest in the Moonflow tutorial battle has slightly different stats. The battle will end after one turn if the game is modified to give Rikku other Special abilities. If the chest is ignored, Rikku's "Woohoo!" line will play the next time a chest is opened or destroyed by her.