An alerted mimic
The mimic is an enemy that disguises itself as a large locked chest. It will sit motionless until the player attempts to open it or hit it with a ranged weapon. It will then show its teeth and attack the player. Once the mimic is defeated, an actual large chest will be spawned that may contain some treasure. They move very fast, do decent damage and have an average amount of health. They are rarely lethal, but can catch some by surprise.
It can be sometimes randomly be summoned through the use of a scroll or wand of create monster. A mimic created this way will act like a chest when first summoned. In order to command it, it must be interacted with and fed items to be tamed. The items will not be consumed, and will merely be held for storage. The mimic makes a great companion due to its speed and apparently unlimited storage, but it cannot be revived once it is slain.
If you encounter a large locked chest in a level you have already explored and do not remember it, try shooting it with a spell or ranged weapon to see if it will come alive and attack you.
While a mimic is not an incredibly tough enemy, one should still be careful when engaging it as escape on foot may not be possible due to its high speed and ability to climb ladders.