Santa's reindeer exist since 1823, except Rudolph, who joined them in 1939. As long as it is not foggy, the reindeer don't have problems to find their route (in the meantime, they know it by heart). If it is foggy, Rudolf helps them with his glowing nose: "Santa Claus's reindeer are a team of flying reindeer traditionally held to pull sleigh of Santa Claus and help him deliver Christmas gifts. The commonly cited names of the reindeer are based on those used in the 1823 poem A Visit from St. Nicholas, arguably the basis of reindeer's popularity as Christmas symbols, where they are called Dasher and Dancer, Prancer and Vixen, Comet and Cupid, and Donder and Blitzen. The subsequent popularity of the Christmas song "Rudolph the Red-Nosed Reindeer" has led to another reindeer name joining the popular roll-call." "Rudolph's story was originally written in verse by Robert L. May for the Montgomery Ward chain of department stores in 1939, and published as a book to be given to children in the store at Christmas time. According to this story, Rudolph's glowing red nose made him a social outcast among the other reindeer. However, one Christmas eve Santa Claus was having a lot of difficulty making his flight around the world because it was too foggy. When Santa went to Rudolph's house to deliver his presents he noticed the glowing red nose in the darkened bedroom and decided it could be a makeshift lamp to guide his sleigh. He asked Rudolph to lead the sleigh for the rest of the night, Rudolph accepted and returned home a hero for having helped Santa Claus." Source and further information: http://en.wikipedia.org/wiki/Santa_Claus's_reindeer