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.