Most religious historians and experts in folklore believe that there is no valid evidence to indicate that St. Nicholas ever existed as a human. In fact, there are quite a few indicators that his life story was simply recycled from those of Pagan gods. Many other ancient Pagan gods and goddesses were similarly Christianized in the early centuries of the Church. His legends seems to have been mainly created out of myths attributed to the Greek God Poseidon, the Roman God Neptune, and the Teutonic God Hold Nickar. The Christian church created a fictional life history for St. Nicholas. He was given the name Hagios Nikolaos (a.k.a. St. Nicholas of Myra).
Many legends and miracles are attributed to Saint Nicholas. When he was an infant, his mother only nursed him on Wednesdays and Fridays; he fasted the remaining days. During his lifetime, he adored children and often threw gifts anonymously into the windows of their homes. A sailor who fell overboard was reputedly saved by Nicholas when the saint walked on water, retrieved the sailor and carried him back to the ship. After an innkeeper had robbed & dismembered some students, Nicholas reputedly re-assembled them and restored them to life. Nicholas took pity on a poverty-stricken family with 3 daughters who faced the threat of being forced into prostitution because they had no wedding dowries. For two daughters he crept-up to their house at night and threw bags of gold through a bedroom window. For the last daughter, he threw a bag of gold down the chimney -- which landed in a stocking she had set by the fireplace for drying. The traditional association of chimneys & stockings with Santa Claus comes from this story. Nicholas was also noted for his generosity with children -- he would reward them with treats if they had studied their catechism & behaved well. Nicholas was therefore patron saint of schoolchildren & sailors.
The transformation of Saint Nicholas to Santa Claus happened largely in America -- with inspiration from the Dutch. In the early days of Dutch New York, "Sinterklass" became known among the English-speaking as "Santa Claus" (or "Saint Nick"). In 1809 Washington Irving, a member of the New York Historical Society (which promoted a Dutch Saint Nicholas as its patron saint), created a tale of a chubby, pipe-smoking little Saint Nicholas who rode a magic horse through the air visiting all houses in New York. The elfish figure was small enough to slide down chimneys with gifts for the good children and switches for the bad ones.
Santa Claus is the sum total of several trends, customs and beliefs that only got unified about a century and a half ago. His story is told through an ex-animation of the 3 names given to him in America: St. Nicholas, Kriss Kringle and Santa Clause.
Much of the present form of the Santa story is undoubtedly due to the works of Clement Clark Moore and the cartoons of Thomas Nast. In 1822, Dr. Moore from New York wrote a Christmas poem, "A visit from St. Nicholas" to read out to his children on X'mas Eve. The following year one Ms Harriet Butler read the poem and requested a copy from him. Later she sent it without Dr. Moore's consent for publishing to Troy, New York Sentinel. Consequently it was published and became popular. In 1938 Dr. Moore revealed that St. Nicholas was his creation. And since then it has appeared countless times.
The 19th century American cartoonist Nast who had lived on the same West 23rd Street as Dr. Moore, did a series of Christmas drawings for Harper's Weekly. It was where the today's much familiar fat and rosy cheeked Santa with large beard and ringing bell made his debut after being modified from fat, little elf-like creature depicted in Dr. Moore's poem.
And perhaps what made Santa more realistic is the classic reply of the editor of New York Sun in response to the 8-year old Virginia O' Hanlon's query whether there really was a Santa Claus. The ed replied "Yes, Virginia, there is a Santa Claus', and made Santa living for ever to the kids.