Wasn't there already an explanation in the game? It was said that Majora's Mask was used by an ancient tribe in hexing rituals. Then, when its power proved too great, too malevolent, they sealed it away, and the tribe vanished.
However, it does not say that they created it, only used it. Perhaps they did create it, I don't know. I do remember reading a theory somewhere that said that the evil spirit Majora was created by the Goddesses of Hyrule as punishment for the fact that Termina didn't believe in them. Another theory is that said tribe were the Dark Interlopers in Twilight Princess who created the Fused Shadow and therefore also created Majora (due to the similarity in appearance between Majora's Mask and the Fused Shadow). I'd say the MM/FS theory is pretty solid. I mean, they must have been able to do some powerful magic (and evil too) and both incidents were said to have taken place ages ago, giving leeway in the timeline. So that's where I think it originated.