The Mask of Scents is given to Link by the Deku Butler after finishing his race. After clearing the Woodfall Temple and bringing the Deku Princess back to the Deku Palace, Link can talk to the Deku Butler, who is waiting to meet the hero in the Deku Shrine outside of the palace. Once there, he will ask Link to race him. If Link manages to keep up, the Deku Butler will say how it reminds him of racing with his son, and then gives him the Mask of Scents in return. The mask was also originally owned by his son.
The mask's design is based on specially-trained pigs that are used to sniff for truffles, a rare fungus.
Mask of Scents in Majora's Mask
Link obtaining the Mask of Scents in Majora's Mask
↑"It seems our butler has taken somewhat of a liking to you, Mr. Link. I believe you'll find him waiting in the shrine that will be to your right when you leave the palace." — Deku Princess (Majora's Mask)
↑"Actually, when I see you, I am reminded of my son who left home long ago... Somehow, I feel as if I am once again racing with my son..." — Deku Butler (Majora's Mask)
↑"My son often used that mask to find mushrooms in the forest. Oh, how I long for those days." — Deku Butler (Majora's Mask)
↑"What's that? You want a Blue Potion, do you? Well, you gave me a mushroom, so I'll give you one for free." — Kotake (Majora's Mask)
↑"The power of this mask allows you to see what you can smell..." — Tatl (Majora's Mask)