Mystery boxes are boxes that contain unknown items that correlate with what the theme of the box is. It can be clothing, furniture, flow, coin or gem boxes. These stay on ourWorld for a longer time than Monthly boxes, and are not usually rare. They can still be sold for a significant amount of money at the Marketplace, depending on what kind of item you get.
The current Monthly Mystery Box for January 2017: (which will be updated every month)
The Bento Mystery Boxes from 2016 are still available also: