Below Gwal-y-Filiast the Afon Taf snakes through a wooded valley, and in the river is a naturally created hollow called Crochan Arthur (Arthur’s cauldron).
Wirt Sikes mentions it in ‘British Goblins’ (1880):
This [feature] is under a cromlech at Dolwillim, on the banks of the Tawe, and in the stream itself when the water is high; it is a circular hole of considerable depth, accurately bored in the stone by the action of the water. This hole is called Arthur’s Pot, and according to local belief was made by Merlin for the hero king to cook his dinner in.
Perhaps Arthur kept his dog in Gwal-y-Filiast (the lair of the greyhound bitch) to keep her out of the way while his dinner was stewing.
Think these folk associated fire with water, to moderns that is simply incomprehensible, but this possible association survived up until the Iron Age, for example Ceridwen's cauldron beneath Bala lake?
Also:-
themodernantiquarian.com/post/5645/folklore/twlc_y_filiast.html#comments