Heru-ur (also called Harmerti) is the oldest version of Horus, and was a falcon creator-god who was known for restraining Apep. His eyes were the sun and the moon; during a new moon, he was blind and was called Mekhenty-er-irty ("he who has no eyes") and open the return of his sight, he was called Khenty-irty ("he who has eyes"). While blind, Horus was quite dangerous, sometimes attacking his friends after mistaking them for enemies. He was a son of Geb and Nut and was the patron god of Letopolis.
As a child, Horus was called Har-pa-khered ("Horus the child" and called Harpocrates by the Greeks) and was a son of either Osiris and Isis or Banebdjetet and Hatmehit. He was depicted as a naked boy with a finger in his mouth, sitting on a lotus with his mother. In this form, he was a fertility god and was depicted with a cornucopia. Har-pa-Khered became very popular during the time of the Roman Empire, when he was depicted riding a goose or ram (note Banebdjetet, his father, was a ram god).
Later Horus became absolutely aligned as a son of the dead body of Osiris and Isis (alternatively: he emerged from Saosis' acacia tree). This is very often cited as "the" Horus in many scholarly works. In truth, this Horus was called Har-sa-iset or Harsiesis.
As Behedti, Horus was the patron deity of Behdet (cur: Edfu), where he was strongly associated with the falcon.
Later still he became associated with the sun god Ra where they combined especially at Heliopolis and became Ra-Herekhty (also Ra-Heru-akhety, Her-akhety ("Horus of the two horizons"), Har-em-akhet ("Horus upon the horizon"), Horakhety, Harmachis (Greek)), god of the morning sun.
In the 3rd millennium BC, Set became the patron god of the pharaohs (replacing Horus in the form of Har-mau or Harsomtus) after overthrowing Seth in the form of Har-wer or Haroeris, but as the story of Set's murder of his brother became popular, Har-mau was switched back. A war between Set and Horus ensued, lasting for eighty years. Har-mau tore off one leg and the testicles of Set, who in turn took out Har-mau's left eye (hence he is referred to as "the one-eyed god"). His eye was later returned to him. Horus won the war (with the support of Neith) and became the ruler of Lower and Upper Egypt. Seth was castrated or killed or moved in with Re and became the voice of thunder.