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.

On occasion, a small mimic poses as a spellbook so persuasively that it actually is a spellbook item, and you can pick it up and carry it around. Attempting to identify, read, throw or kick the book reveals it as a mimic. (If mimics have been genocided, this silently doesn’t happen.)

Internally this is stored using the book’s otrapped flag.

Small mimics (but not large or giant ones) can mimic the floor, looking like nothing in the same way a trapper or lurker above does.