Mimic

The Mimic bites the hand that opens the chest

How to run it. 58 Hit Points, AC 12, Speed 20. As a Bonus Action it takes the shape of a Medium or Small object: a chest, a door, a chair. In object form anything that touches it sticks, Grappled with escape DC 13, and checks to escape have Disadvantage.

Why the grapple matters. Against a Grappled target the Bite gains Advantage and deals 2d8 + 3 Piercing plus 1d8 Acid, 16.5 on average. Against AC 15 that takes its hit chance from 55% to 80%. Getting free is hard: at Athletics +5 with Disadvantage, a character escapes 42% of the time; at +0, 16%. Whoever touched the chest will be there a while.

The choice that matters is the disguise. A treasure chest is the joke everyone knows. A door the party must open, or the one chair in a room, lands harder because it breaks a rule they didn't know they trusted.

The common mistake is over-using it. One mimic makes players check every chest for a session. Three mimics make them stop opening chests, and that kills the fun of finding treasure. Use it rarely and let real treasure follow.

For players. Touch things with a pole or a thrown pebble first. If someone is stuck, the others should hit the Mimic: 58 Hit Points at AC 12 goes down fast, and it is immune to Acid, so bring anything else.

The human angle. Schultz, Dayan and Montague showed dopamine neurons fire on the gap between reward expected and reward received, a prediction error. A chest promises reward, so the brain lights up before the lid moves. The Mimic flips that signal: the bigger the expected loot, the harder the surprise bites.

Source: Schultz, W., Dayan, P., & Montague, P. R. (1997). A neural substrate of prediction and reward. Science, 275(5306), 1593–1599. doi:10.1126/science.275.5306.1593