Mari Lwyd
Though it may sound more like the name of a pleasant Welsh accountant who enjoys a spot of brandy on occasion, Mari Lwyd is actually a rather macabre tradition that occurs late in the year, around the Christmas season.
It entails groups of people galavanting about the neighborhood, singing jolly carols — that’s the normal part — with one of the party members being disguised in a robe while brandishing the skull of a horse. That’s the slightly less normal part.
Obviously, Animal Crossing has never explicitly referenced Christmas, opting instead for Toy Day, the home-brand version that probably breaks as soon as you take it out of the packaging. This leaves a perfectly reasonable chunk of time before New Year’s celebrations for Mari Lwyd to make its horrifying debut.
This would be especially chilling if you had an equine villager like Colton or Reneigh, who would undoubtedly be murdered in order to become the eponymous holiday horse. A worthy sacrifice, I’d say; they’re both jerks, and would be much better off repurposed as a festive singing ghoul.