Monsters shouldn’t be able to hide under the iron chain attached to you when you’re punished.

Implement hoard scarabs from D&D. They are a x, which can hide under object piles containing at least 25 gold pieces (and only object piles like that; they can’t hide under single non-gold objects or furniture or anything; possibly they can only hide under a gold-containing pile when the gold is on the top). They have a nasty poisonous bite.

When the game wants to generate one in a random location, it either creates a gold pile for the scarab to hide under where it generates or alternatively it searches around the level for a suitable gold pile and puts the scarab there. (If it can’t find one, it may just fail to generate).

They may also pop out at you from inside randomly generated gold-containing chests, possibly as a container trap effect.

If the player shoots or zaps something and there is a submerged or hiding monster the player doesn’t know is there in the way, it just flies past without interacting.

Objects thrown at hidden monsters (whether the monster is hiding under something or an undetected mimic) pass right through the square as if no monster is on the square.

When a monster that hides under objects moves from one hiding spot to another, there’s a very low chance that you notice it and it becomes unhidden. This chance is boosted by higher Wisdom, and having searching provides a boost.

Monsters that hide under objects can also hide under or behind various types of terrain, such as sinks, ladders, thrones, and gravestones.

Scale M1_CONCEAL with monster size, so that larger monsters with M1_CONCEAL can hide behind objects of a sufficiently high weight or those that are oc_big.

Fairies, a n monster similar to pixies: very small, flying, capable of hiding under objects. However, they are lawfully aligned and (?) always generated peaceful. If you chat to a non-hostile fairy, it will heal you a moderate amount (as in the Legend of Zelda) and become cancelled (a cancelled fairy can no longer heal you).

Pixies, an n monster that can hide under objects. Chatting to them will make them say “Hey! Listen!” as an Ocarina of Time reference.

Special message when a self-concealing pet “reluctantly hides under” a cursed item instead of stepping over it. Or maybe pets which hide under objects will never hide under cursed objects.