Hen Wen first appears in the Welsh Triads, the medieval collection of Welsh folklore and traditional history preserved in the White Book of Rhydderch and the Red Book of Hergest. She is named in Triad 26, "The Three Powerful Swineherds of the Island of Britain," a triad that celebrates the three greatest pig-keepers in Welsh tradition. The first is Pryderi son of Pwyll, Lord of Annwfn, who tended the swine of Penndaran Dyfed in Glyn Cuch and could not be deceived or forced by anyone. The second is Drystan son of Tallwch, who tended the swine of March son of Meirchyawn and kept every piglet safe even when Arthur, March, Cei, and Bedwyr tried to steal them. The third is Coll son of Collfrewy, who tended the swine of Dallwyr Dallben in the Glen of Dallwyr in Cornwall, and among his swine was one that was pregnant. Her name was Hen Wen, and a prophecy had foretold that the Island of Britain would be the worse for the womb-burden she carried.

King Arthur, hearing the prophecy, assembled the army of the entire Island of Britain and set out to seek and destroy her. Hen Wen fled, and Coll ran after her, his hand on her bristles, following her wherever she went on the sea or on the land. At Penrhyn Awstin in Cornwall, she entered the sea, and Coll followed her into the water. She swam across the channel and re-emerged on land at Aber Tarogi in Gwent Is-coed, a subdivision of the kingdom of Gwent. There she began to give birth, not in one place but scattered across the landscape, each birth leaving a permanent mark upon the land.

At the Wheat Field, Maes Gwenith, in Gwent, she dropped a grain of wheat and a bee. From that day to this, the Wheat Field in Gwent has been the best place for wheat and for bees in all Britain, a bounty born from the belly of a fleeing sow. At Llonion in Pembroke, she dropped a grain of barley and a bee, or in some versions a grain of wheat, and from that day the barley of Llonion has been proverbial for its quality. These gifts were the benign face of her womb-burden, the blessings that the land received even as Arthur's army pressed close behind her.

But the darker births came on the Hill of Cyferthwch in Arfon, in the mountains of Eryri that men call Snowdonia. There Hen Wen gave birth to a wolf-cub and a young eagle. Coll gave the eagle to Breat, a prince of the North, and the wolf-cub to Mergaed, and both men were the worse for them. The wolf and the eagle became creatures of destruction, the predatory offspring of a prophetic pig, harbingers of the ruin that the prophecy had promised. Finally, at Llanfair in Arfon, beneath the Black Rock called Maen Du, Hen Wen gave birth to a kitten. Coll seized the small black creature, this ball of scratching fur and claws, and hurled it from the rock into the Menai Strait, the narrow stretch of turbulent water that separates the Welsh mainland from the Isle of Anglesey. He hoped it would drown. But the kitten clung to life with the same ferocity it had shown in clawing at Coll's hands. It swam across the strait and washed ashore on Anglesey, where the sons of Palug, a local chieftain, found it and took it in. They raised it in ignorance of the doom it carried, and it grew into Cath Palug, the monstrous cat that became one of the Three Great Oppressions of Anglesey, devouring nine score warriors, one hundred and eighty skilled fighters, before Sir Cei pierced it with his sword and a polished shield.

The tale of Hen Wen has been interpreted in many ways. Some have seen her as a figure of primal fertility, the white sow as mother-goddess whose wanderings across Britain map the sacred geography of the land, each birth a founding myth for a place. Others have suggested that the story is allegorical, that the sow represents a female religious leader or a symbol of an older faith that Arthur, the champion of the new order, sought to destroy. In the modern imagination, Hen Wen has become a clairvoyant pig, most famously in Lloyd Alexander's Chronicles of Prydain, where she is a prophetic animal kept by the enchanter Dallben and the swineherd Coll, hunted by the Horned King for her power to see the future. But in the original Welsh tradition, she was not clairvoyant. She was prophetic only in the sense that her womb carried the future of Britain, a future of wheat and bees, of wolves and eagles, and of a black cat that would devour an army. She was the white sow who ran across the land and left it changed forever, the mother of monsters and meadows alike, the proof that prophecy is not a voice but a birth, and that the womb-burden of a single animal can be the destiny of a kingdom.