A new mimic-class monster that can imitate an object (such as a scroll, potion, or wand) so deeply that it actually exists as that object, and you can pick it up and move it around without it revealing itself. It will only reveal itself, which destroys the object and creates the monster in its place, when you or some other monster attempts to use it (or possibly when it would stack with some other object, because an object that is a mimic clearly should not stack with regular objects).

Alternatively, just give this feature to small mimics.

Internally, this would probably work by generating a mextra struct on the object to store the details of the mimic.