This is what i thought.
I copied these from Zelda Wiki just because I didn't feel like writing out my theories. These are only three, but since they're already typed out, I thought "eh, why not?". These helped my theories along, so I'm just posting them to sum me up.
Separate Entity
Majora (ムジュラ, Mujura) was an evil being plaguing Termina in ancient times. Though the game doesn't give much specific history on the origins of Majora's Mask, it does state that in ancient times, the mask was used by an ancient tribe who used it in rituals involving the casting of hexes and curses, until they finally sealed it away from the world in the hopes that its evil power would never be unleashed again.
Hundreds of years later, it was sought out and found by the Happy Mask Salesman. Soon afterwards, the Skull Kid scared the salesman unconscious and stole the mask from him and began to access its power, awakening the consciousness of the mask.
The Skull Kid, wearing Majora's Mask, still doesn't take on Majora's mind or personality; he is simply possessed by the Mask. At the end of the game, inside the Moon, Majora manifests out of the mask into Majora's Mask, then Majora's Incarnation, an insane being that dances around the boss chamber. After the Incarnation is defeated, Majora's transformation is completed, and the Incarnation becomes Majora's Wrath. Majora is eventually defeated, and the being itself disintegrates. The mask is left behind, cleansed of the evil inhabiting it.
The Twili
The Fused Shadow, with an eye of Majora for comparison
It has also been suggested by some that the "ancient and evil" tribe is perhaps referring to the ancestors of the Twili, the Dark Interlopers, a race whose existence was revealed in Twilight Princess. This is due to similarities between an ancient object created by the Twili dubbed the Fused Shadow and Majora's Mask, as they share similar powers and they have a similar design. Also, Zant's fighting style in the eighth dungeon of Twilight Princess is similar to that of Majora's Mask, in fluidity, style, and spontaneity.
Majora's Mask takes place in an alternate universe. However, Majora's Mask could be a parallel version of the Fused Shadow.
The "ancient and evil" tribe, however, could also be the Sheikah, due to their symbol being an eye similar to the eyes of the Majora's Mask. This, however, leaves a gap of information on why they would hex and curse others, as they were devoted to the goddesses. On the other hand, it could explain why the Sheikah were called the "shadows of Hylians," and additionally why the mask is marked in the manner it is. This opinion is, yet again, left to fan speculation.
The Sheikah
Another theory suggests that the Sheikah may be the creators of the Fused Shadow and the ancestors of the Twili, given their affinity with shadow. This means that the Sheikah's Terminan counterparts, who were physically absent from Majora's Mask, very much like many of the Sheikah were absent from Ocarina of Time, were the creators of Majora's Mask; given, of course, that it is the Terminan counterpart to the Hylian Fused Shadow. With this established, their counterparts may very well be the Ikana, with Ikana Canyon being the obvious Terminan counterpart to Kakariko Village, sporting similar geographical features, a graveyard, and other similar landmarks.
Despite these similarities, however, there has been no confirmation of a connection between the two by Nintendo, and any perceived connection is merely fan speculation.
I think another thing was that Fierce Deity and Majora were great enemies and the reason that Majora (child) willingly gave up the mask was so that he could face Fierce Deity in a Final Battle to the death.