Hope it's acceptable.
After comparing the stats of the existing creatures I came up with these suggested stats.
class : spirit
atk 260 //hits like monks
def 39 //between black slime and elder skeleton
HP 200 //Black slime range
Speed - 0.4 // Being shaped like a chest isn't helping it's speed.
Size - 32x32 96x128
Drops - //as sentient treasure chests they could be rich in money, drop low level swords, armor, boots, various (honestly kind of useless) equipment.
If I'm figuring it right this would make them ideal in early caves, and dungeon areas ie Kobold side of Semos Mines,
Thanks for your time.