Creeper

The Creeper is an eye creature that is part of the Brain of Cthulhu boss fight. They ONLY drop items when Brain of Cthulhu is alive. Furthermore, they won't drop hearts if they player is already at maximum health. They are immune to the Confused debuff.

Trivia
This coud be a reference to the enemy in the block placing game, Minecraft

The above text is reference to the trivia on the Blood Moon page.