There is a really nice ancient fairy tale, The Magical Welsh White Cow, which relates the legend of the origin of Welsh Black cattle. The notion sort of seems fantastical and magical that a white cow took herself away and her only remaining offspring forever turned black -- "Whereupon not only did the elfin cow arise and go home, but all her progeny to the third and fourth generations went home with her, disappearing in the air over the hill tops and returning nevermore. Only one cow remained of all the farmer's herds, and she had turned from milky white to raven black..."
But however magical the notion, I've long theorized it was a story based on actual history. The following exerpts from old texts well supports an historical basis for the old story. Breeders should also note that in the description the White Welsh had black spots, and that the old Dynevor herd was considered to be White Welsh cattle.
"Farm live stock of Great Britain", c.1907, By Robert Wallace, Loudon M. Douglas, Primrose McConnell, W. B. Wale
"The ancient white breed of the Principality has been rehabilitated by selecting and mating together those specimens of the South Wales breed which have "thrown back " in the matter of colour to their forest ancestors. In every other respect they are distinctly Pembroke cattle. Their colour is chiefly white, but there are frequently black spots over the body. The muzzles, ears, and eyelashes are black, and the feet and fetlocks should also be black. Charles Mathias, of Lamphey Court, to whom the Welsh originals of Plates X. and XLIII. belong, has raised this off-shoot of the South Wales breed to a position of importance."
"The difficulty of procuring at all times stud bulls good enough to keep up the standard of a small number of cattle is got over by the use of a choice black bull when a white is not available. A large proportion of his progeny take after the mothers in being white with black points, and those that are born black are transferred to the black cattle herd."
"R. H. Harvey (i S74) says: "The late Lord Dynevor had some very fine specimens of the white breed near Llandeilo, and I have often admired the five-year-old oxen as I have passed the park." For Professor David Low's beautifully illustrated book on The Domesticated Animals of the British Isles, published in 1842, was selected a Pembrokeshire "cow eight years old," from Haverfordwest, to represent the type of the Wild Forest breed. The painting of the animal is among the collection of original oil paintings which were used by him to illustrate the book, and which now adorn the walls of the Agriculture Department of Edinburgh University."