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 you by the Deku Butler, but only if you beat his race. After clearing Woodfall Temple of Odolwa, talk to the Deku Butler. He'll tell you to meet him in a cave outside of the palace (to the right as you leave the palace). Once there, he will ask you to race him, a race that is far easier with the Bunny Hood equipped. If you manage to keep up, he will say how it reminds him of racing with his son, and gives you the Mask of Scents.