The Catbox of Schrodinger's cat is found in the Elona+ mod of Elona. This is the same item previously named Schrodinger's Cat, which had the same appearance and effect as the Gem stone of Mani (if you already had such an item in regular Elona before upgrading, it retains the old look and function in Elona+, but newer instances will show the updated look and will work correctly). The name is a reference to Schrödinger's cat, a thought experiment that attempted to ridicule the apparent ridiculousness its author saw in the early theories of quantum superpositions. The item itself can be opened, and will either produce a cat, or a zombie cat.
It has a 50% chance of dropping from a quantum creature.
The item sprite ID for the catbox is 710 (Row 21, Column 17) using the item sprite zero-based position system.