The Golden Key spawns on the ground, at random, in any of the Forest stages in Megabonk. It only shows up if you haven't unlocked the character Monke, since the key is part of what you need to free it and make it playable.
This article is not about the item name Key that you get from chests. That item gives you a 10-percent chance to open future Chests for free, and it has nothing to do with unlocking Monke.
