Although farriery as a profession was not established as a guild in England until 1160, under Baron de Fer, horseshoes as such were used even in Caesar's days. The Worshipful Company of Farriers is still going in Hereford, training up masters in the art to shoe the horses of today for their various tasks.
Unfortunately iron does not last well, so few examples of early horseshoes-or sandals, as they were indeed called-have been preserved. However, it is true that a blacksmith was an extremely important craftsman, since he made weapons for defense as well as other important tools.
It is also very true that the Libyan horses that Arthur is reputed to have bought in the horse fair at Septimania would have had foot problems, carrying the weight of large, well-armed men and moving over surfaces different to the ones in their native country. Though Hollywood would have us believe fifth-century knights wore full-body suits of armor, they actually used only breastplates, and leg and arm guards such as the Roman soldiers had, and carried heavy swords and shields. That gear alone required big, strong horses to bear them any distance. Such weight, as well as the uneven and wet terrain, would have caused hoof problems. Britain in those days tended to be wetter and warmer than it is now.
I feel it is reasonable that horseshoes would have had to be developed for the purpose of keeping Arthur's cavalry sound. "No hoof, no horse" is still a farrier's truism. And there would have needed to be men knowledgeable about hooves and iron, to make such aids.
It is historically true that someone like Artos, a Comes Britannorum and dux bellorum (war leader), existed in the latter part of the fifth century and the early part of the sixth century. This leader united the tribes of Britain to defend themselves against the Saxons invading from their base at York. I prefer to keep to the historical facts, such as they are, in this extrapolation. These facts include the strong Roman and Christian religious practices current in those times.
I have not included Merlin in this story because he is not historically mentioned by contemporary chroniclers, namely Gildas and Nennius, who are reputed to have lived when Artos did. Nor have I bothered resetting the age-old triangle of Arthur, Guinevere, and Lancelot. Arthur is not thought to have married before the first of his great battles, when his fame would have made him a fine match. Guinevere is purported to have come from a prestigious family of Roman Celts, and marriage with Arthur would have enhanced her family's reputation as well as shown support to his efforts.
Since this is my story, about a facet of those times, I can deal with such facts as I choose from those available. But all the farriery details have been checked by Master Farrier Joseph Tobin, Associate of the Worshipful Company of Farriers, and those facts concerning horses in general by my daughter, Georgeanne Kennedy, Irish Certificate of Equine Sciences and British Horse Society Assistant Instructor.
The reading I did on my own.
Dragonhold-Underhill Wicklow County, Ireland, 1995