The Mysterious Package was an unobtainable pet summon, which was supposed to summon a pet drone that would drop a book every few minutes. It was likely intended to be available to those who bought Terraria on Amazon. It was only obtainable through hacks and was removed entirely in the 1.1.5933 update.
- The pet resembles Amazon.com's planned Amazon Prime Air drones, as seen in their promotional material: http://www.amazon.com/b?node=8037720011
- The pet's buff has the Elf Pet buff icon.
- The item and pet has a sign on the package that represents the Amazon logo.