But Moore originally wrote it as Donder instead of Donner...why the change I have no idea.
And I educate myself, from Wikipedia:
In An American Anthology, 1787–1900, Edmund Clarence Stedman reprints the 1844 Clement Clarke Moore version of the poem, including the German spelling of “Donder and Blitzen,” rather than the original 1823 version using the Dutch spelling, “Dunder and Blixem.”[1] Both phrases translate as “Thunder and Lightning” in English, though German for thunder is now spelled Donner, and the Dutch words would nowadays be spelled Donder and Bliksem.