I recently read a lavishly illustrated book about midwinter folklore, THE FRIGHT BEFORE CHRISTMAS, by Jeff Belanger, featuring Krampus, the Yule Cat, Belsnickel, and many other Christmas-season monsters; however, it also covers some benevolent creatures such as La Befana, Saint Nicholas, and of course Santa Claus. Terrors lurk in the longest, darkest night at the coldest time of year. In the past, telling frightful tales at Christmas was a British tradition. Even now, a popular Christmas song mentions "scary ghost stories" along with caroling in the snow. Dickens's A CHRISTMAS CAROL is just the best known. Our preindustrial ancestors recognized the frightening aspect of midwinter; that's why the lights, fires, bells, feasting, and evergreens exist in the first place. They ward off the darkness and keep the demons at bay. Some of the Yuletide boogeymen used to serve as shadow counterparts of Saint Nicholas, punishing naughty children while he rewarded nice ones, in a sort of bad cop / good cop partnership.
Nowadays we joke about getting coal in stockings from Santa if we haven't been "good" (sometimes with the contemporary angle that coal might be a reward instead of a punishment when energy costs rise). Saint Nick's old-style sidekicks or substitutes, though, would beat naughty children with sticks, haul them away in sacks to an unspecified fate, or eat them. On the other hand, if you're lucky you might get a visit from the Italian witch Befana, who may sweep your house in addition to leaving gifts for children. The animated film THE NIGHTMARE BEFORE CHRISTMAS beautifully highlights the traditional solstice ambiguity of the festive combined with the monstrous. Likewise, in Terry Pratchett's HOGFATHER Death himself fills in for the Hogfather (Discworld's Santa) when the latter is temporarily unavailable.
A long time ago in an online writing group, I read a story about an alternate world in which Santa is a frightening figure who comes down the chimney at midwinter to perpetrate terrible acts. From a certain point of view, a mysteriously omniscient man who constantly watches you from afar and sneaks into your house in the middle of the night regardless of locked doors DOES sound sinister.
Ellen Datlow's new anthology CHRISTMAS AND OTHER HORRORS explores the dark side of the winter solstice in a variety of stories featuring Christmas and other seasonal celebrations and customs. Some of the horrors are based on actual folklore, others created by the individual authors.
Speaking of HOGFATHER, here's a link to my favorite quote from the entire Discworld series, Death's explanation of why human beings need myths and fantasies:
We Need Fantasy to Be HumanMargaret L. Carter
Please explore love among the monsters at Carter's Crypt.