Back in 2012 Scooby Doo: Mysteries Incorporated did the Wrath of the Krampus. The next year Grimm showcased him in the episode Twelve Days of Krampus and American Dad did Minstrel Krampus.
I'd like to see those become classics, like Charlie Brown or Rankin & Bass. Here are a few other holiday specials I'd to the list.
Batman: The Animated Series, Christmas with the Joker
Ren & Stimpy's, A Scooter for Yaksmas.
Invader Zim, The Most Horrible Xmas Ever
Scooby Doo!, Haunted Holidays and Scooby Doo Christmas both have awesomely freakish snowmen.
Courage the Cowardly Dog, The Nutcracker
and of course my all time favorite that I've covered before Billy & Mandy Save Christmas
Love this CN Christmas promo with Courage the Cowardly Dog.