The Horseman's Word
Jason Ogg, the smith in Lancre, knows the secret of the Horseman's Word. Confronted with the stud stallions, the red-eyed and foam-flecked kings of the horse kingdom, the soup-plate-hoofed beasts that had kicked lesser men through walls, Jason invariably quietly closes the forge doors, and a little while later leads out a strangely-docile and newly-shod beast.
As there are no secrets from witches, Granny Weatherwax has extracted the secret of the Horseman's Word from Jason. It boils down to:
"Well, ma'am, what happens is, I gets old of 'un and smacks 'un between the eyes with the hammer before'un knows what's happening, and then I whispers in his ear, I sez, 'Cross me, you bugger, and I'll have thy goolies on t'anvil, thou knows I can'"
Jason has applied the Word to a unicorn stallion prior to fitting it with silver shoes.
On the other hand, Miss Healstether mentions a book about the "Horseman's Word" describing a secret society of horsemen who accomplished the same end by means of a potion, similar to the Pictsies' bird-taming oil. This was able to overcome even the beasts' fear of Orcs. There's more than one way to shoe a horse.
Tiffany Aching picked up a Horseman's Word which was demonstrated to be able to immobilise horses in I Shall Wear Midnight. Perhaps Jason uses a different method or (gasp) fobbed Granny Weatherwax off?
Annotations
The Horseman's Word was in fact a Scottish secret society of farmhands, but it does involve speaking a secret word (as it was a group of men it was probably a rude one). The legends also speak of "riding the Devil's horse".
The Society of the Horseman's Word was a fraternal secret society that operated in Scotland from the eighteenth through to the twentieth century. Its members were drawn from those who worked with horses, including horse trainers, blacksmiths and ploughmen, and involved the teaching of magical rituals designed to provide the practitioner with the ability to control both horses and women. It also acted as a form of trade union, aiming to gain better rights for its members.
The initiation rituals into the society incorporated a number of elements such as reading passages from the Bible backwards, and the secrets included Masonic-style oaths, gestures, passwords and handshakes. Like the similar societies of the Miller's Word and the Toadsmen, they were believed to have practiced witchcraft. In East Anglia, horsemen with these powers were sometimes called "Horse Witches".