A ring of monster attraction: Each individual ring is associated with a randomly selected specific, common monster species (or possibly monster class), and causes monsters of that species/class to generate disproportionately often while it’s worn. When the type is known, the ring appears as “ring of monster attraction”, but when an individual one is fully identified, it ring appears as e.g. a “ring of newt attraction”.

Depending on the potential for abuse, it should probably not be allowed to wish for specific monsters. E.g. it’s probably bad to allow the player to wish for a ring of wraith attraction.