Interlopers

The Dark Interlopers are a tribe of sorcerers spoken of in The Legend of Zelda: Twilight Princess. The Dark Interlopers were most likely among the combatants of the Hyrulean Civil War, in which many races fought amongst themselves to establish dominion over the Sacred Realm and the Triforce. The Light Spirit Lanayru tells Link about these beings and their history after he restores the Vessel of Light.

Biography
Using the dark power of the Fused Shadow, the Dark Interlopers tried to establish dominion over the Sacred Realm, but were stopped. As a punishment, the Goddesses sealed them away into the Twilight Realm, and split the Fused Shadows, the source of their evil magic, into four separate shards. At the same time, the other races that fought in the Hyrulean Civil War settled the dispute in a seemingly peaceful manner. The Dark Interlopers would eventually evolve into the Twili race of beings.

Sheikah
Some fans go as far to say that the Sheikah are the Dark Interlopers from whom the Twili are descended; being so close to the Royal Family, they would know much of the Triforce. Both the Dark Interlopers and the Sheikah have red eyes and both are dark magic practitioners. Another clue to this is the the Sheikah symbol at the back of the Fused Shadows. However, the presence of Impaz and the fact that the Sheikah protected the Hylian royalty would seem to contradict this and thus make it improbable for the two to be the one and same, but the term "interlopers" may suggest that a group of Sheikah interfered with the loyal Sheikah, whose duty it was to protect the royal family, and wiped them out to get to the Triforce.

The Legend of Zelda: Majora's Mask
Another race, though not featured in the series and is only briefly mentioned in The Legend of Zelda: Majora's Mask, are those that used Majora's Mask in their 'hexing rituals'. There are many similarites between the Twili and Majora's Mask itself, which implies that the Twili could be the same race. The similarities involve some shapes featured on Majora's Mask being present on Midna's helmet, the Fused Shadow (the eye especially). Zant's behavior is very similar to creatures specific to Majora's Mask and the Mask itself, such as Zant screeching in a similar fashion to Majora's Mask, being a sympathetic villain much like Skull Kid who wore the Mask. During the mention of those who used the Mask, the Happy Mask Salesman mentions that the mask was hidden in shadows, which the Twilight Realm is sometimes called (to be precise, "The Realm of Shadows"). Also, the entrance to the Stone Tower Temple in Majora's Mask appears to be a large face with it's tongue sticking out. Zant's helmet also resembles a face with it's tongue out, which would lead to the belief that the ones who built the tower (believed to be the same race as the ones who made the mask) could very well be the Twili (also connecting to the Garo, the protectors of Ikana Canyon and the Stone Tower).

Gerudo
Another possibility is that at least a portion of the Gerudo tribe were the ones who gained the mysterious power of the Fused Shadows and were cast into the Twilight Realm. This would be supported by Midna's story that the Twili race were originally a tribe of thieves, and why the entrance to the Twilight Realm is located in the Gerudo Desert. The Gerudo are no longer present in Twilight Princess, and just before Zant is fought within the Palace of Twilight, Midna mentions that the Twili's ancestors "lost their king to greed" - implying that she means Ganondorf, whose desire to posess the Triforce eventually led to his defeat. As for appearance, Midna does have the classic red-orange hair of the Gerudo and her true form is tall and slender, just as the Gerudo women once were. Also, there is a Gerudo pattern and converted Gerudo symbol on the front of Zant's robe, which he wore even before his encounter with Ganondorf, whom he worshiped as a god. Gerudo also worship Ganondorf as a god, which a gossip stone tells you. It is also striking that Veran, who is called a Gerudo Sorceress, is nearly identical in design to Midna.

Zuna
The Twili bear a strong resemblance to the Zuna, and even dress in a similar fashion. Due to the Mirror of Twilight's desert location, and the Zuna's desert home, they could have at least some relation. The ancestors of the Zuna were also skilled in dark magic, as they created the Trident of Power. It is possible that the Dark Interlopers were not only ancestors of the Twili, but of the Zuna as well.

Other
Finally there remains the possibility that the Dark Interlopers were not of any one race or tribe, but were a cult of people made up of several different races and cultures. Evidence to support this is Lanayrus vision where Link kills his closest friend to seek after the Triforce, only to become a Dark Interloper himself. This could be an interpretation that several people throughout Hyrule betrayed their fellow countrymen to obtain the Triforce and eventually united as the Dark Interlopers. One sealed in the Twilight Realm, the long term effect of the Twilight on them could have transformed them into one single race, and may explain the large differences in appearance that some Twili have from others.