“It seems hardly probable that the custom commenced at an early date, but its origin is veiled in obscurity.” (Evans, pg. 94)

The Wikipedia article is sourced as “The Times: Mon 31 March 1997”, but actually links to the Flags of the World website, which includes a lengthier extract from the Times article, as quoted in a post on the old FOTW usenet group…so that’s not really the greatest chain of authority (and definitely breaks Wikipedia’s citation rules, but that’s a separate problem…)

This article has two suspicious statements, one that de Ferrers family “gave its name to farriers” and that the “privilege” was granted by William the Conqueror all the way back in the 11th century.

This may very well be an accurate description of the local tradition but neither of those things are likely to be literally true. Henry de Ferrers certainly participated in the conquest in 1066, and was one of the major landholders in England afterwards. His name comes from the village of Ferrières in Normandy; the village was presumably named after a nearby iron mine or forge (from the word for iron in Latin, “fer”). A farrier is an ironsmith who makes horseshoes, so the names are indirectly related, but the de Ferrers family wasn’t a family of farriers, and the word farrier doesn’t come from their name.

There isn’t really any evidence that William the Conqueror (or anyone else) granted the de Ferrers family this strange privilege. The first evidence we have of it comes from 17th-century antiquarians. According to William Camden, at Oakham,

“…hard by the Church, which is large and faire, remaine the crackt and decaying wals of an old Castle which Walkelin de Ferrariis built in the first times of the Norman kings. And that it hath beene the dwelling place of the Ferrars, besides the credit of writers and generall report, the great horse shoes, which in times past that familie gave in their armes, formed upon the gate and in the hall, may sufficiently prove.”

Later in the 17th century, James Wright also noted that

“The Lord of the Castle and Mannour of Okeham for the time being claims by prescription a Franchise or Royalty very rare and of singular note, viz., That the first time that any Peer of the Kingdom shall happen to pass through the Precincts of this Lordship, he shall forfeit as a Homage a Shoe from the Horse heron he rideth, unless he redeem it with mony. The true Originale of which Custome I have not been able on my utmost endeavour to discover. But that such and time out of mind had been the Usage appears by several Monumental Horseshoes (some Gilded and of curious Workmanship) nail’d upon the Castle Hall Door.”

The earliest horseshoe, or at least the earliest one stamped with a legible name and date, was only from 1602, although there were “some more antient, whose inscriptions are now hardly legible.”

John Evans suggested the de Ferrers eventually forgot their origins in the village of Ferrières. Instead they claimed descent from a farrier, and adopted the horseshoe as their emblem. But this is only speculation; there’s no way to be sure where the custom came from, only that the horseshoes were there in the 17th century and some of them seemed to be very old.

Sources:

Marios Costambeys, "Ferrers, Henry de (d. 1093x1100)", in Oxford Dictionary of National Biography (Oxford University Press, 2004)

William Camden, Britannia (1607), trans. Philemon Holland

James Wright, The History and Antiquities of the County of Rutland (1684)

John Evans, “Notes on the Horse-shoe Custom at Oakham, Rutlandshire”, in Proceedings of the Society of Antiquaries of London, 2nd series, vol. XIV (1893)