The Mabinogion of Wales
The Mabinogion is not one myth but a collection of stories gleaned from the traditions of Welsh bards and storytellers over many centuries. They were passed from one bard to another until anonymously collated in the 12th century. Later versions were written down in the White Book of Rhydderch (now in the Welsh National Library) and the Red Book of Hergest (now preserved at Oxford University). Even then they remained largely unknown outside (and even inside) Wales until Lady Charlotte Guest translated them in 1849.
The Mabonogi (the name means alternately "a story for children" or "a bard's tale" depending on whose translation you prefer) are comprised of 4 branches, entitled "Pwyll", "Branwen", Manawydan", and "Math". Aside from these four branches there are another 8 individual tales in the British and French style. Taken together, these tales of heroes and stirring deeds depict a Celtic vision of enchantment and romance that moves effortlessly between the physical landscape of Wales and the Celtic underworld.
In some of the later stories King Arthur appears, though certainly not in the guise of the chivalrous knight known to modern readers. He is a giant, whose deeds involve ridding Wales of witches, monsters and other giants with aid of - no, not twelve knights, but his own band of hags, witches, and monsters.
To give you a flavour of the Mabinogion, I've included short summaries of several popular tales. Click on one of the links below for more.
Olwen of the White Track
Pwyll and Rhiannon
Branwen invades Ireland
Manawyddan and the mouse
Gwydion and Llew
Maxen Wledig, Emperor of Rome