Mask of Scents

The Mask of Scents is a pig face-shaped mask from Majora's Mask. It imbues the wearer with the ability to sniff for the scent of and find Magic Mushrooms, which are given to Kotake to create Blue Potion. Because Blue Potions can be bought elsewhere (although for more Rupees), and given the obscurity of the mushroom's location, this is widely regarded as a relatively useless mask; however, Link can receive a free Blue Potion in exchange for the Mushroom.

Obtaining the Mask
The Mask of Scents is given to Link by the Deku Butler, but only if he beats him in his race. After clearing Woodfall Temple of Odolwa, Link needs to talk to the Deku Butler. He'll tell the young hero to meet him in a cave outside of the palace. Once there, he will ask Link to race him, a race that is far easier with the Bunny Hood equipped. If Link manages to keeps up, he will say how it reminds him of racing with his son, and gives him the Mask of Scents in return.