Ho ho ho! Merry Christmas!
The best known (at least in modern times) mascot of Christmas, developed in the United States as an amalgam of the story of St. Nicholas of Myra and various other seasonal folk heroes, with many aspects provided by the classic poem A Visit From St. Nicholas (popularly known by its first line, 'Twas the Night Before Christmas).
The Santa Claus myth is based largely on the Dutch holiday of "Sinterklaas" (a hastily pronounced "St. Nicholas", who comes down the chimney on the 5th/6 December) and the imagery of the Saint in question carried over to his North Pole incarnation. In the original stories, Sinterklaas was accompanied by black slaves; these have become demons (The Krampus) in German-speaking culture, and friendly elves in the USA. In the Netherlands, the black companions are nowadays portrayed as St. Nicholas' friends and employees. Note that in several countries in Europe, Sinterklaas and Santa Claus are now considered two entirely different characters, each with their own elaborate holiday. It should also be noted that his transition from badass Turkish saint to "jolly old elf" was influenced by another winter gift-giver: Odin.