Traditional Christmas characters here in the United States are typically friendly. Think of the many incarnations of St. Nicholas. Iceland on the other hand is home to a character that sounds terrifying. The Yule Cat, or Jólakötturin, is described as a huge and especially vicious cat who lurks about the snowy countryside during Christmas time eating people who have not received any new clothes to wear before Christmas Eve. He is the house pet of Gryla and her sons who are terrifying in their own right. They are a tale for another post another time.
Though referred to as an ancient tradition, written accounts of the Yule Cat have only been located as recently as the 19th century. The threat of being eaten by the Yule Cat was used by farmers as an incentive for their workers to finish processing the autumn wool before Christmas. The ones who took part in the work would be rewarded with new clothes, but those who did not would get nothing and thus would be preyed upon by the monstrous cat. The cat has alternatively been described as merely eating away the food of ones without new clothes during Christmas feasts. The perception of the Yule Cat as a man-eating beast was partly popularized by poems of Jóhannes úr Kötlum.
Now aside from the Krampus, this thing is probably the scariest bit of xmas folklore i have ever run accross. Not in any small part due to the fact that I HATE cats.