Ganon
The scourge of Hyrule, Ganon has over the centuries repeatedly tried to conquer the nation, his desire for the Triforce keeping him alive. While Links and Zeldas have come and gone, Ganon has always endured, his hatred and desire providing him strength to continue on in his quest to rule first Hyrule, and then the world.
Ganon: The Evil Boar Sorcerer
Taking on the form of a goat-hoffed, boar-headed demonic monster, Ganon possesses the power of a demigod when at his full power. It takes an incredible amount of power to seal him away, and besides the Master Sword, the only thing that can truly harm him are the Silver Arrows.
Definition: Ganondorf (The Gerudo King): transformed by the Triforce of Power
Aliases: Agahnim, Ganondorf, "King of Evil", "Prince of Darkness", etc.
Alternate Spelling: Gannon (Mistranslation from The Legend of Zelda)
Appears in: The Legend of Zelda, in his demonic form; Indirectly in The Adventure of Link; A Link to the Past, in his demonic form; Four Swords Adventures, in his demonic form; Ocarina of Time (and Master Quest), as his original form, Ganondorf Dragmire; The Oracle games, Oracle of Ages and Oracle of Seasons, if you play a linked game; Aliased as Ganon, in The Wind Waker; And in an unknown role in Twilight Princess.
See Ganondorf for the history of the man behind the demon.
Ganon in The Legend of Zelda
A long, long time ago the World was in an age of Chaos. In the middle of this chaos, in a little kingdom in the land of Hyrule, a legend was being banded down from generation to generation, the legend of the 'Triforce'; golden triangles possessing mystical powers.
One day, an evil army attacked this peaceful little kingdom and stole the Triforce of Power. This army was led by Gannon, the powerful Prince of Darkness who sought to plunge the World into fear and darkness under his rule. Fearing his wicked rule, Zelda, the princess of this kingdom, split up the Triforce of Wisdom into eight fragments and hid them throughout the realm to save the last remaining Triforce from the cluthes of the evil Ganon. At the same time, she commanded her most trustworthy nursemaid, Impa, to secretly escape into the land and go find a man with enough courage to destroy the evil Gannon. Upon heraing this, Ganon grew angry, imprisoned the princess, and sent out a party in search of Impa. - taken from ZeldaUniverse
So begins the first tale of Ganon's quest to conquer Hyrule. In this tale, he possesses the Triforce of Power, and seeks the Triforce of Wisdom as well. Imprisoning Zelda, he has made his stronghold in Death Mountain, and awaits Link to come and bring him the Triforce of Wisdom.
Ganon in The Adventure of Link
While never truly appearing as himself, Ganon does eventually appear. It has been prophecised that, were Link to be sacrificed, and his blood sprinkled upon the ashes of Ganon, Ganon would return to life. In the end, Ganon is revived, but in the form of Link's shadow. Essentially Dark Link, Ganon then proceeds to destroy in turn the one who destroyed him.
Ganon in A Link to the Past
Possessing the entire Triforce, Ganon has become the undisputed ruler of the Dark World. Seeking to return to his old plans, Ganon seeped back into Hyrule through the cracks in the seal. Taking on physical form, he took on the name of Agahnim, and slowly ingraciated himself into a position of power within the court of Hyrule, before staging a coup de'etat against the Royal Family. Eventually, after having picked off his minions and alter-ego, Link comes face to face with the King of Evil, who now wields the united might of the whole Triforce. In the end, Ganon is defeated by the power of the Silver Arrows, and loses the completed Triforce to Link.