Really, St. Nicholas didn't celebrate Christmas and probably never saw, or knew about, reindeer because he lived in what's now Turkey. The Dutch legend Sinterklaas traveled on a gray horse and wore bishop's robes.
Thomas Nast is generally credited with “inventing” the image popularly recognized as Santa Claus when he first drew him for the 1862 Christmas season Harper’s Weekly cover to memorialize the family sacrifices during the Civil War. But Nast’s Santa was not a "jolly old elf", rather he was melancholy, sad for the separation of soldiers and families.
In any event, the idea that Nast “invented” Santa Claus overlooks the centuries-long antecedents to his invention.
In 1823, for example, a more cheerful fellow was depicted in an anonymous poem entitled, "A Visit from St. Nicholas" (now popularly known as "The Night Before Christmas"). The poem appeared in a Troy, New York newspaper depicting Santa as a jolly fellow who rode in a sleigh drawn by eight reindeer. The author, it later was revealed, was Clement C. Moore, a well known professor of biblical learning in the General Theological Seminary in New York from 1821 to 1850.
Regardless of where he came from, our modern day Khris Kringle is largely a product of Coca-Cola. In 1931 the company decided to give him a makeover. Before that time, Santa Claus appeared as anything from a green elf to Nast's somber St. Nicholas to even a gaunt figure in animal skins. The company hired an artist, Haddon Sundblom, to create a fatherly fellow with a billowy beard, extensive girth, rosy cheeks...and a bottle of Coke, of course. Each year thereafter, a new painting helped ingrain the image in American, and worldwide, culture.