The Guardian Acorn is an item from Link's Awakening.
The Guardian Acorn is a power-up in Link's Awakening that reduces the amount of damage Link takes in half for a limited time. This power-up is dropped by enemies in the overworld and can not be acquired any other way. A different tune will play while Link has it, and will go away when he loses it.
Guardian Acorn Sprite from Link's Awakening.