Anybody know the source of these critters’ names?

Try the very popular English children’s poem “A Visit from St. Nicholas” anonymous 1823 that accounts for Santa’s eight “original” reindeer:

Now Dasher!  Now Dancer!  Now, Prancer and Vixen!
On, Comet!  On, Cupid!  On, Donner[sic] and Blitzen!
To the top of the porch!  To the top of the wall!
Now dash away!  Dash away!  Dash away all!

Unfortunately Dashell Hammet’s purported authorship has to be ruled out by a well attested 1894-1961 lifespan.  Draft reindeer number nine came much later in typecast Rudolf the Red-Nosed Reindeer, first created as promotional gimmick by Montgomery Ward employee Robert L. May in 1939.

I guess I forgot they were in that poem. You are always so on the ball, bandito, but I guess I was reminded of DONNER. from the enty on Wotd under ‘Blunderbuss”, mentioning
Donder, but as you say , it turns out to be Donner.  Danke!

Received text as of 1844 “Donder” and “Blitzen” excuse me!

You’re excused:  Donder or Donner. I sure hope he bring me something on his sleigh: I been wery good.