Majora

Majora was an evil being plaguing Termina in ancient times. Majora's Mask itself was created from the remains of the beast that was Majora, which was in fact itself created by the same tribe. Its recently been debated that the tribe responsible for creating Majora is in fact the Twili, as Midna and her armor both bare a notable resemblance to the final mask. It is still not known for certain what Majora look like, but it is referred to as a beast, similar to the reference to Wolf Link and Pig Ganon as beasts. However, it may resemble the final form of Majora's Mask, or may resemble the form taken by Midna when she uses the Fused Shadows (the game seems to give focus to the eye on her helmet, which is the same as Majora's Mask's) to access Hyrule Castle. The form bears resemblance to Majora's Incarnation. In the manga version, the fierce diety himself defeated Majora by just talking him into, well, dancing. This dancing led to Majora to his demise and leaving nothing but his magic armor behind. The Fierce Diety took a feather from his head and turned it into a sword, then took a piece of the armor and carved out the mask, sealing away the power of Majora.

Adding to speculation, a short text that is presented as a plot point the game, Majora's Mask, states "...And the Gods had a friend..." this insinuates that Majora is in fact a Deity, with God like powers. It is also believed that Majora is to the world of Hyrule what the Devil is to Christianity.

The Skull Kid, wearing the Mask of Majora, still doesn't take on Majora's mind or personality, he is simply possessed by the Mask, created from the beast's remains. At the end of the game, inside the Moon, Majora manifests out of the mask into Majora's Incarnation, an insane being that dances around the boss chamber. After the incarnation is defeated, Majora's manifestation is completed, and the incarnation becomes Majora's Wrath. Majora is eventually defeated, and the being itself disintegrates, but leaving the mask behind, cleansed of the evil inhabiting it.