Old Shaking Chest

Once Skeletron is dead, the cavern layer starts producing something that looks like an ordinary container until it starts bouncing toward you like a slime. The encounter is harmless in both directions: it cannot hurt you and your weapons do nothing to it. The real risk is that something else in the cave kills it on the way over, and only one exists in the world at a time, so nothing replaces it while it lives. A Lifeform Analyzer calls it out before it reaches your screen.

Activate it while carrying a Golden Key and it unfolds into the Elder Slime. That trade happens once per world, and the hopping chest never spawns again afterward. Nothing is permanently lost, since the slime comes back on its own after you kill it, but meeting one with no key in your bag means walking away and hoping it survives until you return. Keep a spare on you while caving and the situation never comes up. It is also the only thing in the game that turns up both inside the dungeon and out in the open world.