One of my personal Appendix N reads (mine, not Gary’s) is Susan Cooper’s The Dark is Rising Sequence. The final book, Silver on the Tree, has the last of the Old Ones, Will Stanton, and Bran Pendragon off hunting a magic sword in the sunken lands on the coast of Wales. There, they are confronted by one of the sword’s guardians, an undead unicorn, Bran called the Mari Lwyd. According to wikipedia, the name means Grey Mare, which could be taken to be Grimoire. The custom of pantomiming a dead horse seems to have originated as part of a Yuletide Wassailing tradition, but what amounts to an undead faerie creature surely belongs around Samhain as well. So naturally, I am using it as my costume for the Village Halloween Parade this year.
In the story, it was destroyed by “a flying may tree” moths that had been flocked on a hawthorne bush. The moths may well have been the souls of its prior victims, much like those that fought the boggarts in Charles deLint’s Tapping the Dream Tree.
I would love to build some stats, but right now I don’t have the time… although I would start by taking the wight template from Fantasy, the horse from Basic, and add in the descriptors Faerie and Undead, give it Terror and a vulnerability to spriitual attack, not sure how to parse that.
And I don’t have time to really flesh out this dead horse because I am working on delivering moire art to my patrons. You can be one too!