Ancient Tribe

The Ancient Tribe was the civilization spoken of by the Happy Mask Salesman in Majora's Mask. According to legend, the tribe used the wicked power of Majora's Mask in hexing rituals. The Mask Salesman continues that the ancient ones sealed the mask into the darkness, so that it might never be misused. The Ancient Tribe evidently vanished, leaving only the wicked Majora's Mask as their legacy.

Builders of the Stone Tower
The location of the Tribe's existence is unknown, but some believe that evidence would suggest that the they built and lived within the Stone Tower Temple. This includes that the structural foundation is seemingly built by an ancient culture, and many aspects of the temple seem to pay tribute to the mask. While the exact cause of their disappearance remains unanswered, it is hinted that they were the opposing force to the Ikana Kingdom, and may have been wiped out in the war. This would also support the idea that not only did the Ancient Tribe inhabit the Stone Tower, but that Majora's Mask had been sealed there, as Igos du Ikana stated the doors to the tower had been opened.

Identity
The exact nature and identity of the Ancient Tribe is not elaborated on in-game. However, it is possible that this tribe is shown elsewhere in the series.
 * This tribe may in fact have been a group of Sheikah or their ancestors, as the Sheikah are shown to have great magical power and prowess, as well as an affinity for shadow and darkness. The Ancient Tribe was also stated to have vanished, and indeed, the Sheikah do not seem to exist nor have any kind of influence in Termina, and even in Hyrule have dwindled almost to the point of extinction.
 * Another possibility is that this tribe was the Interlopers shown in, as again, this group is shown to have much magical power and an affinity for dark magic. Also, the Ancient Tribe used Majora's Mask, while the Interlopers created the Fused Shadow, and the two objects share some similarities, particularly with their eyes. In this case, the tribe's disappearance coincides with the Interlopers being sealed in the Twilight Realm.
 * Still another possibility is that the Ancient Tribe was or was connected to the Dark Tribe spoken of in . Again, this tribe is implied to have had a fair bit of magical skill and, given their name, an affinity for darkness. Moreover, the Dark Tribe was abruptly sealed within the Dark Mirror after their incursion into Hyrule, and thus disappeared with little trace, just as the Ancient Tribe did.
 * The Ancient Tribe could also potentially be the Zuna, who are also shown in Four Swords Adventures, or their ancestors. Though the tribe is never mentioned to have any kind of magical power, they are descended from a race that watched over the Pyramid deep within the Desert of Doubt, inside which sits the Trident, another powerful and evil weapon. This tribe has also never made an appearance in another game as yet, and are fairly isolated and are endemic to a small part of the desert. This could coincide with the disappearance of the Ancient Tribe, and also opens up the possibility that this tribe is somehow connected to the Trident.
 * It is also conceivable that the Ancient Tribe shares connections with all of these aforementioned tribes, perhaps being the common ancestors of them all, or otherwise influencing their ancestors.
 * Alternately, the Ancient Tribe may have been Terminan counterparts of one of said tribes.