In many Celtic-based traditions there is the enduring legend of the battle between the Oak King and the Holly King. These two mighty rulers fight for supremacy as the Wheel of the Year turns each season. At the Winter Solstice, or Yule, the Oak King kills the Holly King, and then reigns until Midsummer, or Litha. Once the Summer Solstice arrives, the Holly King returns to do battle with the old king, and defeats him. The Holly King them rules until Yule.
In some Wiccan traditions, the Oak King and the Holly King are seen as dual aspects of the Horned God. Each of these twin aspects rules for half the year, battles for the favor of the Goddess, and then retires to nurse his wounds for the next six months, until it is time for him to reign once more.
Often, these two entities are portrayed in familiar ways - the Holly King frequently appears as a woodsy version of Santa Claus. He dresses in red, wears a sprig of holly in his tangled hair, and is sometimes depicted driving a team of eight stags. The Oak King is portrayed as a fertility god, and occasionally appears as the Green Man or other lord of the forest.
Ultimately, while these two beings do battle all year long, they are two essential parts of a whole. Despite being enemies, without one, the other would no longer exist.