Eye Symbol

The eye symbol is found in a number of Zelda games, and is often associated with the Sheikah tribe.

Ocarina of Time
This is where the tear-drop eye symbol first appeared. From the beginning it had it's basic form, an eye in the middle with three triangular lashes on top and a tear drop coming down from the pupil. It was the proud symbol of the mysterious Sheikah, the shadow race of Hyrule. The symbol seems to have been very important part of Sheikah culture. They placed it all throughout the sacred Shadow Temple, and on their artifacts, including the Lens of Truth, the Gossip Stones, and the Mask of Truth. They also seem to have commonly placed it on their attire, as both Impa the Sheikah sage of Shadows and Sheik (Princess Zelda disguised as a Sheikah) placed it on their clothing.

The Wind Waker
In The Wind Waker, a version of the eye symbol can be seen at the door of the grounded ship atop Forsaken Fortress, where Ganondorf dwells. Although the tear is bigger and the eye smaller, it is still very close in appearance to the Sheikah eye symbol. Why such a symbol would appear there is a mystery, although it may indicate that the pirate ship where it is found was once crewed by Sheikah-descended pirates. Forsaken Fortress is known to have once been a pirate hideout, though not much is known of them. The eye symbol also appeared in the room containing the Master Sword, where it is displayed with only one triangular eyelash instead of the usual three in the stained glass window of Impa above her head.

Baito, the mail service employee found on Dragon Roost Island, has a tattoo on his back that resemble the Sheikah eye symbol.

Twilight Princess
Another appearance of the symbol would be with the Twili in Twilight Princess. It can be found on their artifacts, such as Midna's helmet and the Fused Shadow. It is also seen on their buildings and in places such as above Zant's throne, the cloak worn by Princess Zelda, and on the forehead of Madame Fanadi. Also, the eye symbol features prominently on the Howling Stones, in the same way as the Gossip Stones from Ocarina of Time. The Symbol is also displayed on the back of Princess Zelda's black cloak, showing the continued link of the Royal family of Hyrule and the Sheikah tribe.

A Link to the Past
Eye symbols have been seen throughout the Zelda series ever since the release of A Link to the Past. In that game, the evil wizard Agahnim has the eye symbol on his robe and throughout his lairs. The symbol has lashes on the top and bottom and the inside circle is so thin that it almost appears feline.

In the opening of the game, it is seen that the Master Sword has a similar eye symbol on the hilt.

In A Link to the Past, Agahnim appears as though from nowhere to quell the misfortunes that are plaguing Hyrule using his powerful magic. Agahnim's origins are not entirely known, though he becomes close to the Royal Family of Hyrule following his curing of Hyrule's ailments before revealing his true colors. What is so curious about him however is the symbol that represents him, which is a modified version of the Eye Symbol that bears an uncanny resemblance to the Sheikah insignia. The emblem appears in front of the doors to Hyrule Castle Tower, Agahnim's lair in Hyrule Castle, and again in front of the topmost chamber of Ganon's Tower, where Link faces the dark wizard for a second time before discovering that he was Ganon's alter ego. This symbol, combined with Agahnim's relationship with the Royal Family and his knowledge of magic, has given rise to speculation that Agahnim may have had some connection to the Sheikah tribe, though this also is unconfirmed. Ganon refers to Agahnim as his alter ego, casting doubt on this possibility, because Agahnim may have simply been a disguise Ganon used to fool the Royal Family and not a genuine Sheikah or member of any other race, though the true aspects of Ganon's relationship with Agahnim remain partially unclear.

Oracle of Ages
In Oracle of Ages, it is noted that the villain of the game, the Sorceress of Shadows known as Veran, also bears a symbol resembling an eye that is slightly similar to the Eye Symbol of the Sheikah; along with Gerudo Symbols on her shoulder-pads. Her self-proclaimed title, "Sorceress of Shadows", also correlates with the fact that the Sheikah also had magic based upon the element of shadows. Veran's red eyes also match the color of the Sheikah, and these facts have caused some to speculate that she may have some connection to either the Sheikah, the Twili, or both. But as with Agahnim, apart from this symbol on her gown and her ability to use "shadow magic", Veran's relation to either or both of these groups has been left up to debate since no mention is made by her regarding either race. As such, Veran's true race remains unknown to date, leaving much of her backstory shrouded in mystery and leaving her origins open to speculation.

Four Swords, Four Swords Adventures, and The Minish Cap
During Four Swords, the evil wind demon Vaati is seen as personifying the symbol with his one giant stylized eye. It was seen again during Four Swords Adventures and The Minish Cap. Vaati's origins are revealed to be rooted with an entirely different race from the Sheikah, that of the tiny thumb-sized Minish race. The villain first gained power through the magic of the Minish Cap, a creation of his master Ezlo, in The Minish Cap. He then took on the form of a Hylian-like sorcerer and chose a single eye as his symbol of power.

What is so interesting about the form Vaati took after his transformation from a Minish however is that not only did he now have pointed ears, but he also retained red eyes (unlike other Minish featured in the game) as well as white hairs (like the only confirmed Sheikah in the series). These traits have caused some to believe that Vaati's new appearance and symbol were perhaps based upon the Sheikah, though this remains unconfirmed by any official source. The Sheikah do not make any official appearance in this game, and no evidence exists to suggest Vaati even knew about the Sheikah. This is further complicated by the fact that Vaati is revealed to be of Minish descent. Still, due to the similar appearance he took on after seizing the Minish Cap and his choice of an eye as his symbol, his knowledge of the Sheikah or any connection he may have to them is also left up to debate.

However, since the Sheikah are known for their skill in magic and as the only real Sheikah known is a female, it is intersting to note that Vaati is supported by a group of female mages in Four Swords Adventures. One of them, in the Village of the Blue Maiden stage, is a Fortune Teller like Madame Fanadi, a suspected Sheikah from Twilight Princess.

Trivia

 * The Sheikah eye symbol was likely inspired by the Ancient Egyptian Eye of Horus or Wedjet symbol. This eye was sometimes used to represent the falcon god Horus, the serpent goddess Wadjet or the mother goddess Hathor. It was used as a symbol of protection for the Pharaoh and was worn as a medallion to ward off black magic and curses.
 * In Phantom Hourglass, the symbol on the front of the Guard Notebook resembles the Eye Symbol.