Quetzalcoatl (Aztec) was one of many names of the same God, others were
Gukumatz,the Quich, Nine Wind, and Kukulcan. the earliest known reference to the plumed serpent God is Toltec, though there is a school of thought that says the Olmecs worshipped the plumed serpent.

Quetzalcoatl was a man, king, God and demi god, depending on which legend you go by. According to legend, Quetzalcoatl was involved in the creation and destruction of the Four Suns, a succession of eras and or worlds. At the end of the Fourth Sun, Quetzalcoatl was forced to flee the city of Cholula for Tlapallan.

one wonders if the Incas were influenced by the Aztec or Toltec, considering the parallel's between Varroche and the Plumed serpent legends. particularly "then he went to the sea coast. Thereupon he fashioned a raft of serpents. When he had arranged (the raft), there he placed himself, as if it were his boat. Then he set off going across the sea. No one knows how he came to arrive there at Tlapallan." (Florentine Codex)"

anway here is the reason stated by Cortes for being considered the Plumed serpent.

"For a long time we have known from the writings of our ancestors that neither I [Motecuzoma], nor any of those who dwell in this land, are natives of it, but foreigners who came from a very distant land and likewise we know that a chieftain, of whom they were all vassals, brought our people to this region. And he returned to his native land and after many years came again, by which time all those who had remained were married to native women and had built up the villages and raised children. And when he wished to lead them away again they would not go, nor even admit him as their chief; and so he departed. And we have always held that those who descended from him would come and conquer this land and take us as their vassals. So because of the place from which you claim to come, namely from where the sun rises, and the thing you tell us of the great Lord or king who sent you here, we believe and are certain that his is our natural Lord, especially as you say that he has known of us for some time." (Hernan Cortés, Letters from Mexico, translated by A.R. Pagden, (New York: Grossman Publishers, 1971), 85-60.)

Torquemada expounded on this, but it is unknown whether he based the passage on any actual Texts.

"He said of those who dwelt in the city of Cholula, they held for certain that in coming times were to come form the Sea towards the rising sun, white men with white beards, like him, and that they would be Lords of these lands, and that they were their brothers. And in this way the Indians always awaited the fulfillment of this prophecy, and when they saw the Christians, they called them gods, and brothers of Quetzalcoatl." (Torquemada, 1943, 2:51 and Mendieta, 1971, 92-93.)

there is however still skepticism in academia that Cortes was ever really considered a God by the Aztec.

"As to the well-aired notion of Cortés being supposed to have been the god Quetzalcoatl returning, the suspect first portion of Book 12 contains the only such references in the Nahuatl corpus (to the best of my knowledge)." James Lockhart, The Nahuas After the Conquest, (Stanford: Stanford University Press, 1992), 20.

regardless of what I may or may not believe, there was enough justification for the Spanish to absorb and rewrite native myths and traditions. In point of Fact it was SOP for the Catholic Church to do just so. It allowed political and religious control and acceptance by the native populations.

though I am Sure I have read References to the plumed serpent god being white, with blond hair. I can not in all fairness say I can locate them at this date, nor can I vouch for the veracity of the references I saw so long ago.