Goblin Peon

Peons only come with a goblin invasion. They are the rank and file of it, second in number to the warriors alone, and nothing else in the game produces one.

Most ground enemies open a door and walk through. The peon takes the door apart instead, and the wreckage lands on the floor as an item you can pick up and hang again later. That is not a permanent loss, but for the rest of the fight the entrance is open and whatever was behind the peon follows it inside.

It is shorter than your character, which matters more than it sounds. A gap two blocks high reads as sealed while you are standing in front of it, and a peon strolls straight through. Check the height of every hole in an arena wall before the invasion lands, not during it.