Rust Monster Pen is a fighter Monster Room that will be introduced with Boss Monster 2. It is noted as being a "cycling" room, meaning a player can discard a card to activate its effect.
This card works well with cards that allow a player to draw more cards, and works when the player has excess Monster Room cards they don't need. This card is a bit of a thorn in the side to players who have a Trap Room.
While the -1 might seem minimal, this card also pairs well with Werewolf Den.