Across rural Wales — Glamorgan, Pembrokeshire, Carmarthenshire, and right up into the lanes of Gwynedd — there are old stories of a creature that no traveller wants to meet. Gwyllgi, the 'Dog of Darkness'. A black hound the size of a calf, with eyes that glow like hot coals and breath that stinks of the grave.
He is not a pack hunter like his cousin Cŵn Annwn. Gwyllgi walks alone. He is met on lonely roads, at crossroads, on the path home from the inn after dark. He does not chase. He does not bark. He simply appears, falls in step beside you for a stretch of road, and then is gone.
Some who saw him reported nothing worse than a terrible dread that lingered for days. Others were struck dumb, or fell ill, or never quite came right again. The most famous Welsh telling, recorded in the 19th century, describes a man near Newport who met a Gwyllgi on a country lane and was paralysed with fear until cockcrow.
There is debate about what he is. Some folklorists call him a guardian of liminal places — a creature whose job is to mark the boundary between this world and the next. Others say he is a damned soul, condemned to walk the same stretch of road forever. The country people themselves rarely speculated. They simply did not walk those lanes alone after dark.
Gwyllgi belongs to a wider British tradition of black dogs — Black Shuck in East Anglia, the Barghest of Yorkshire, the Cù-Sìth of the Highlands — but the Welsh version has a particular weight to him. He is silent. He is patient. He has all night.
If a calm, enormous black dog ever pads up beside you on a country road in Wales after sunset, the old folk would tell you exactly what to do: keep walking, do not speak to him, and do not, under any circumstance, look him in the eye.