Why is that, you might ask?
Well, in the original poem, the names of two of the reindeer were DUNDER and BLIXEM, which are the Dutch words for Thunder and Lightning. However, Clement Moore was German, and when he reprinted the poem in 1844, he changed those names to DONNER and BLITZEN, which are the German words for Thunder and Lightning.
So there you have it!
Now Dasher! Now, Dancer! Now, Prancer and Vixen!
On Comet! On Cupid! On Donner and Blitzen!
And of course, Rudolph, but his is a story for another time! Ho, ho ho!!!