Cthulhu

The image to the right is a concept sketch of  done by Tiy. It was previously thought that the development team intended to add the boss in an update, but they revealed it was a troll, and they will never add a Cthulhu boss in terraria. Ever.

Trivia

 * The concept of Cthulhu was originally devised by author H. P. Lovecraft. The creature is, today, a popular element of Lovecraftian Horror.
 * He is missing one of his eyes. This is because of the Eye of Cthulhu, which was destroyed earlier on.
 * As of the 1.2 update you can fight the Brain of Cthulhu.