The Mabinogion is a collection of eleven old Welsh stories. In the form in which these stories survive, they are medieval, and some of them are probably in fact medieval stories (two of them are very old King Arthur tales). Some of the stories, the Pedair Cainc y Mabinogi (the "Four Branches of the Mabinogi"), are likely older, and contain old Celtic myths in medieval cross-dress.
The Mabinogion is great in its own right, and is also a well to which many fantasy writers over the years have had recourse for inspiration and/or outright theft (see, in particular, Lloyd Alexander's Chronicles of Prydain). If you're stuck, you might do the same.