From ‘British Goblins’ by Wirt Sikes, 1880:
"The Gwyllion are female fairies of frightful
characteristics, who haunt lonely roads in the Welsh mountains and lead
night-wanderers astray. The Welsh word gwyll is variously used to signify
gloom, shade, duskiness, a hag, a witch, a fairy and a goblin; but its special
application is to these mountain fairies of gloomy and harmful habits, as
distinct from the Ellyllon of the forest glades and dingles, which are more
often beneficent. The Gwyllion take on a more distinct individuality under
another name – as the Ellyllon do in mischievous Puck – and the Old Woman of
the Mountain typifies all her kind. She is very carefully described… in the
guise in which she haunted Llanhyddel
Mountain in
Monmouthshire. This was the semblance of a poor old woman, with an oblong
four-cornered hat, ash-coloured clothes, her apron thrown across her shoulder,
with a pot or wooden can in her hand, such as poor people carry to fetch milk
in. always going before the spectator, and sometimes crying ‘Wow up!’ This is
an English form of a Welsh cry of distress, ‘Wwb!’ or ‘Ww-bwb!’ Those who saw this apparition would be sure to
lose their way…
When people first lost their way and saw her before them,
they used to hurry forward and try to catch her, supposing her to be a
flesh-and-blood woman who could set them right; but they never could overtake
her, and she on her part never looked back; so that no man ever saw her face.
She has also been seen on the Black
Mountain in Breconshire."