Here's a quite wonderful old reference to British White cattle (a matter of opinion, but what other breed was milk white then or is now?) to start off this blog:
'The milk-white milch cow gave enough of milk to every one who desired it; and however frequently milked, or by whatever number of persons, she was never found deficient. All persons who drank of her milk were healed of every illness ; from fools they became wise ; and from being wicked, became happy. This cow went round the world; and wherever she appeared, she filled with milk all the vessels that could be found, leaving calves behind her for all the wise and happy. It was from her that all the milch cows in the world were obtained. After traversing through the island of Britain, for the benefit and blessing of country and kindred, she reached the Vale of Towy; where, tempted by her fine appearance and superior condition, the natives sought to kill and eat her; but just as they were proceeding to effect their purpose, she vanished from between their hands, and was never seen again. A house still remains in the locality, called Y Fuwch Laethwen Lefrith (The Milk-white Milch Cow.)' " http://www.sacred-texts.com/neu/celt/wfl/wfl03.htm