ACME monster trap
Despite what implication the title might give you, the ACME monster trap does not actully trap monsters. Instead, usually with a god's guidance, the artifact is activated and summons a monster for the hero to fight.
It is unknown why it presents itself as a monster trap or a monster fighting device when the reality is far from it. We would ask the company that produced them but when asked, they decided to remain quiet on the manner.