Monthly Treasure Chests are special chance items, sold in the Premium Shop.
They have a fixed price of 5 HC, and can drop many kinds of items randomly like wearables, keepers, noms, etc.
Premium chests have a fixed price of 20 HC.

Most MTC seem to follow a theme for the month they were released. For example,
the Mischievous Treasure Chest, launched on October, had a Halloween theme.

Remember: Only one item is dropped per chest from Standard chests.
Premium chests give all five items.

Note: Bear in mind that there is a good accumulation of treasure chests, so this page is image heavy. Please be patient.