Donner or Donder?
Is Santa's reindeer named Donner or Donder?
Santa's reindeer were first named in "A Visit from St. Nicholas" (better known by its first line, "Twas the Night Before Christmas") in 1823. In the original printing, the final two reindeers' names are Dunder and Blixem, which are Dutch for "thunder and lightning." In an 1844 printing by Clement Clarke Moore, who is credited as being the author, these names are changed to Donder and Blitzen.>
(This figures into a debate over whether Moore really wrote the poem in question, which is too complicated to get into here.)
The 1949 song "Rudolph the Red-Nosed Reindeer" calls them Donner and Blitzen, which are German for "thunder and lightning." This is how they're most widely known today.
For more information, see The Donner Party's Over at the Urban Legend Reference Pages.