Iceland has a disturbing number of violent, dark, and ghoulish Christmas traditions. The cutest of these is Jólakötturinn, the Yule cat.
Jólakötturinn is a giant, festive cat that hunts down people who have not been given new clothing before Christmas, and eats them. This ties in with the Icelandic tradition of giving socks to children in the days leading up to Christmas -- specifically, children who did their chores get new socks, while the ones who did not do their chores get eaten by a cat.
This is generally held to be an ancient tradition, but the first reference to it in writing appears to be from Jón Árnason's second collection of folklore (Íslenzkar Þjóðsögur og Æfintýri, Vol 2) in 1864. There are a number of traditions that change from household to household, but the cat would either eat the child, or steal their food, or steal their food and come back to eat the child if they did not mend their ways. One assumes that the appetite of the cat that came to visit your house correlated strongly with how strict your parents were.
Jólakötturinn is the pet of Grýla and Leppalúður, a pair of nasty giants who have their own unpleasant habits -- Grýla, in particular, likes to eat disobedient children. The Yule Lads, thirteen pranksters that come to visit in the days leading up to Christmas, are Grýla's children.
The internet, being an imprecise memesmith, has twisted this tradition into something rather different: at the moment, Twitter is all aflutter about the Icelandic cat that "eats people who aren't wearing the clothes they got for Christmas". While this is almost right, it suggests an entirely different dynamic, a sort of Christmas-gratitude-enforcing, vengeance-seeking cat. This is entirely incorrect; Jólakötturinn is simply a giant Christmas cat that doesn't like to eat fresh clothing.