Since the dawn of the beginning. The modern "Santa" we all know didn't exist.
Actually, the story that started behind it was the Turkish legend of Bishop Nicholas who "helped poor noblemen provide dowries for their daughters by throwing gold coins down their chimneys-- the coins landed magically in stockings hung by the fire to dry."
Because of this, Nicholas was later canonized as the patron saint of children, among others.
Saint Nicholas's name changed because of the Dutch, who brought the legend to America, called him "Sinter Klaas". (You can guess where did the name "Santa Claus" came from. BLAME THE DUTCH!) XDDDDD
As for the myths and stuff, those were the Americans, and other European countries' faults. XD
For the most part, it was Clement Moore's 1823 poem "A Visit From Saint Nicholas" that made Santa Claus virtually "Exist" within the people's state of mind. Which is really stupid, a poem is a poem. It can either relate to a real thing by using figurative languages, or to emotions and other sorts of devices. XD
I don't see why people would go all through those troubles to prove Santa Claus didn't exist. The answer lies already within history itself. XD