Monsters should be able to activate statue traps. There’s no reason why they should only trigger for the player.

Resolve the quirkiness of statue traps - they are a silent trap on which a real statue potentially containing spellbooks can generate, which you can teleport off the trap to smash for its spellbooks (indicating it’s a true statue) and do not show up with telepathy or monster detection (indicating it’s a true statue), but reveal themselves as “You find a foo posing as a statue” (indicating it’s been a living monster all along). There are two basic options:

1. Statue traps are posing monsters: Remove statue traps as a floor trap. In their place, put real but non-moving monsters that appear as statues of themselves, using the monster appearance system used by mimics. This means that you can see the monsters with telepathy and monster detection and you can’t exploit it to get spellbooks.
2. Statue traps magically animate a statue when someone moves onto their space: Searching for traps may still reveal statue traps, but it does not activate the monster. This implementation might also actually remove statue traps as a floor trap, and instead use the otrapped field on the statue object, so relocating the statue doesn’t affect its trapped status. Trap detection should still show trapped statues.

Statue traps generate monsters with a +5 difficulty modifier (though this might be too high). This is because a statue that surprisingly turns into a level-appropriate monster isn’t usually very threatening.

Randomly generated statues and figurines (and masks in variants that have them) should use a higher effective difficulty when choosing the associated monster species, because using low-level monsters is boring and typically useless to the hero.

This should probably also apply to statue traps, since they are not very threatening at the moment.

Hitting a statue trap with cancellation removes the trap, so that it is just a regular statue.

Level that contains a structure with a lot of gargoyle statues. Some of the statues are statue traps.