I’ve always referred to the person who shod my horse as a “farrier” but over the years, I’ve seen several people write it as “ferrier.” At first, I thought it was an error. But, thinking like my grandfather (who had the largest dictionary I’ve ever seen in his office so you could check the etymology of words), I did a little research.
It turns out that farrier (which is the current preferred English usage) evolved from the Middle French word “ferrier“, which meant blacksmith (back then, iron workers and blacksmiths were one and the same). Ferrier, in turn, evolved from the Latin word ferrarius which means “of iron”, which is from the Latin ferrum, “iron”.
Other spellings throughout the centuries have included ferrer and ferrour.