Majora's Mask is a very mysterious mask, said to one belong to an ancient tribe (which I will talk about) for their hexing and torture rituals. This ancient tribe sealed the mask into darkness, in fear that it will be misused and cause catastrophe in the world. Although it is not mentioned, I believe this tribe created that mask, as no evidence is given that another tribe created it. How it was created is not known, but it must have involved powerful Dark magic, as the mask seems to have control over itself, posses others, and transform (as demonstrated at the end of the game). The mask salesman somehow stumbled upon it in his travels, but it got stolen from him by the Skull Kid.
Now, a question that all want answered and is clouded in mystery is "what is the ancient tribe?". I believe, along with many others, that the ancient tribe is the tribe that raged war in Ikana Canyon against their , the tribe that lived within Stone Tower and Stone Tower Temple, the tribe known as the Garo. The Garo seemed to roam across Ikana Canyon, specifically near Stone Tower. And although some may contradict that the Garo lived in Stone Tower, it is still highly likely that whoever did live in Stone Tower were the ancient tribe. The way the whole temple is designed resembles Majora's Mask. For example take a look at the entrance. It looks like the mask upside-down, and the entrance hall also has a structure resembling the mask. So, although the Garo may have not been the ancient tribe, I believe the ancient tribe lived in Stone Tower. See the video below for some proof.
Although I straightened out some theories about Majora's Mask, there is still a great deal of mystery around it, and it is not likely that any theory can be truly confirmed. Check out the Stone Tower of Babel theory. It is very interesting and believable. Remember to read next post (which will probably be about the Skull Kid), and remember, any questions or post ideas, comment below or email me at firstname.lastname@example.org.