If you have played Majora's Mask for more than a minute, you might have stumbled upon this thing to the left. Tatl gives a small clue as to what it is, but you find out more in the future. It is actually the son of the Deku Butler and the dead body of the Deku Mask's spirit.
After you go to the Deku Shrine and race the Deku Butler, he apologizes for going too fast. He then tells Link that he looks like his son and that he was remembering what it was like to race him. Of course he is implying what you look like with the Deku Mask on. Still need more evidence?
When you play the Elegy of Emptiness as Deku Link, the picture to the right appears as the spirit. This is seen as what the Deku Butler's Son looked like before death. The Goron Mask held the spirit of Darmani and the Zora Mask held the spirit of Mikau, so the Deku Mask holds the spirit of the thing that is created.Finally, the most obvious clue is in the credits. The Deku Butler is CRYING over this distorted tree.
It turns out that the Skull Kid had murdered this poor guy so that he could curse Link with its spirit.