Chester

Chester is a gooey monster disguised as a purple Treasure Chest that appears in Super Mario RPG: Legend of the Seven Stars. There is only one in the game, found behind one of the six doors in Bowser's Keep. When the chest is closed, Chester looks exactly like any regular Treasure Box; this ability allows him to prey easily on carefree adventurers deceived by his mimicry. If Mario hits him from below, Chester will reveal his true form and will engage a battle against Mario's party.

In battle, Chester proves to be a tough opponent: indeed, he is one of the strongest enemies in Bowser's Keep, as he has both a high attack and a high defense. Moreover, he is not affected by status ailments. Surprisingly, although he is met much later in the game, Chester has less HP than his lookalikes Hidon and Box Boy. But Chester makes up for that by resorting to the healing move Mega Recover when he is low in HP. Chester can also use the spells Flame Wall and Sand Storm, that damage all the party members, or simply attack his opponents by stretching his gooey body in order to hit them. Chester can also summon a Bahamutt to help him in battle.

Unlike the other chest enemies from Super Mario RPG: Legend of the Seven Stars, Chester is not fought by hitting a Treasure Chest, rather, he's one of the various monster spawned by Magikoopa.

Trivia

 * Chester's name is a pun on "chest"; "Chester" also happens to be an actual name.
 * Chester's body looks like a beige Ameboid, which implies that they might be related.