Bone Key

The Bone Key is a drop from the Dungeon Guardian. It's used to summon a Baby Skeletron Head pet that follows you until you cancel the buff icon, like other pets.

It can also be noted that using the turtle armor will reflect damage back to the Dungeon Guardian, making it much easier to defeat. At least two people are suggested for this method, but it can be done alone with a Black belt or Ninja gear.

History
Here's a video on how to get the Bone Key without much effort: http://www.youtube.com/watch?v=YzKm9nxLRnA