Magic collar, an amulet without a randomized appearance, which you can apply to a pet to make it wear it, assuming it has a neck. The pet then becomes visible to you at all times while it’s on your level (similar to seeing it with telepathy) and possibly tracked in #overview if it’s not on your level. Farlooking a pet wearing a magic collar shows HP and other additional information as if you had used a stethoscope on it.

Not specified what effects there would be, if any, if the player wears the magic collar.