Drop from Doctor Bones
It looks old and ancient
'Allows you to harness the power of your pets'
By using the "Gold Skull" Item at night (bonesx10 and gold dustx10 at an anvil). You can spawn Dr. Bones repeatedly to farm for the item.
Material in Delta Rune
This item supposedly allows you to use powers from certain pets, (Flandre, Remilla, and a few others)