Rudolph the Red-Nosed Reindeer, but he actually wasn't in the original list of Santa's reindeer names in "Twas The Night Before Christmas."
The classic Christmas poem, first published in 1823 (read the original version here), calls out the following reindeer names:
1. Dasher 2. Dancer 3. Prancer 4. Vixen 5. Comet 6. Cupid 7. Dunder 8. Blixem
Of course, other versions of the names have also been published over the years, including Donner and Blitzen. And Rudolph came later, popularized by the song and film.