This site is said to show the mark of the hoof-print of Arthur's horse. All sorts of people and animals seem to have a penchant for leaving their footprints in rocks. But this poor horse had a reason - he was leaping back from the scary Afanc lake monster, which Arthur was removing from Llyn Barfog. You can see a picture of Llyn Barfog at Mark Scott's 'Susan Cooper's Wales' website the lost land.
The lake also has the following story associated with it:
In a secluded spot in the upland country behind Aberdovey is a small lake called Llyn Barfog, or the Lake of the Bearded One. Its waters are black and gloomy, no fish is ever seen to rise to the surface, and the fowls of the air fly high above it. In times of old the neighbourhood of the lake was haunted by a band of elfin ladies. They were sometimes seen in the dusk of a summer evening, clad all in green, accompanied by their hounds and comely milk-white kine. [An old farmer] had the good luck to catch one of the Gwartheg y Llyn, or kine of the lake, which had fallen in love with the cattle of his herd. [..His]fortune was made. Never was there such a cow, never such calves, never such milk and butter and cheese, and the fame of the Fuwch Gyfeiliorn, or the Stray Cow, spread...
The farmer, who had been poor, became rich... [but eventually] fearing that the elfin cow would become too old to be profitable, he thought that he had better fatten her for the market...
...the butcher raised his red right arm to strike the fatal blow. Just as the bludgeon was falling, a piercing cry awakened the echoes of the hills and made the welkin ring. The butcher's arm was paralysed and the bludgeon fell from his hand. [The green-clad woman on a crag above the lake cried:]
Come thou, Einion's Yellow One,
Stray-horns, the Particoloured Lake Cow,
And the hornless Dodyn,
Arise, come home.
No sooner were these words uttered than the elfin cow and all her progeny [ran for the lake and] disappeared beneath the dark surface, leaving only the yellow water-lily to mark the spot where they had vanished.
From W. Jenkyn Thomas's 1907 'The Stray Cow' in his Welsh Fairy Book.
They climbed on, to the line where the green grassy slope met a grey sky. On the downward sweep of the path on the other side, Barney and Jane were crouched beside a small out-cropping of rock, identical with every other rocky scar on the hill but singled out by a neat slate marker like a label. Will came slowly down the path, his senses open and alert as the ears of a hunting dog, but he felt nothing. Glancing across, he saw the same blankness on Bran's face.
"There's a sort of carved-out circle here that's supposed to be the hoof of Arthur's horse trod - look, it's marked." Barney measured the hollow in the rock with his hand. "And another over there". He sniffed, unimpressed. "Pretty small horse."
"They are hoof-shaped, though" Jane said. Her head was down, her voice slightly husky. "I wonder what really made them?"
"Erosion," Simon said. "Water swirling around".
"With dirt rubbing," Bran said.
Jane said hesitantly, "And frost, cracking the rock."
"Or the hoof of a magic horse, coming down hard, " Barney said. He looked up at Will. "Only it wasn't, was it?"