Small barrel mimic
From A KoL Wiki

You enter the barrel, and attempt to smash a barrel. But what you thought was a barrel turns out to actually be a mimic mimicing a barrel! You're confused -- you thought these things could only mimic chests!
It does a good impression of a monster laying the smack down. Ouch! Ugh! Ouch!
The mimic turns into a reasonable facsimile of your fifth grade teacher, which gives you a stern talking-to. You claw your eyes out in horror. Ugh! Eek! Ugh! Argh! Argh! Ow!
It does a fairly lousy impression of something that's able to hurt you.
It does an awful impression of a monster hitting you in the elbow.
You gain 2 Muscleboundness.
You gain 1 Sarcasm.
Occurs at The Barrel full of Barrels.
References
- Long predating the Dungeons of Doom, this was the first Nethack reference in the Kingdom of Loathing.