Clement C. Moore is generally credited with the poem, but literary scholar Don Foster has pretty convincingly shown that the probable author was Henry Livingston, Jr., another New York poet.
Whoever wrote it, the poem shaped much of what we now "know" about Christmas and Santa Claus--he was a fat, jolly man, for example, and eight reindeer pulled his sleigh.
But the original 1823 poem is a little different from the way we know it:
Now! Dasher, now! Dancer, now! Prancer, and Vixen,Read the above, paying close attention to the commas and exclamation points. The emphasis is all wrong. The more familiar pattern didn't appear for seven years:
On! Comet, on! Cupid, on! Dunder and Blixem.
Now, Dasher! now, Dancer! now, Prancer! now, Vixen!And look at the names of the last two reindeer! They didn't become Donder and Blixen until 1837. (Rudolph showed up in 1939.)
On, Comet! on, Cupid! on, Dunder and Blixem!