Back in the 18th & 19th & early 20th centuries, the UK was home to several odd horse training guilds that mixed mystical rites and secrets with horse training skill and a dash of trickery. One of these, the Horseman's Word, was a guild of horse trainers in Scotland who claimed to have secret words to whisper in a horse's ear to charm him into gentleness. Membership was via initiation; along the lines of various secret societies of the time the ceremonies included midnight meetings, parodies of church ceremonies, secret handshakes, pagan and folkloric elements and so on. Wikipedia has a quite thorough article on this group, and points out how the secrets worked:
Until the initiation ceremony...the horsemen who were not members of the society but potential candidates would have trouble with horses. This would often be caused by older ploughmen who were members of the society tampering with their horses. They would put things like tacks under the horse's collar to cause it to behave irrationally. .... Foul substances placed in front of the horse or on the animal itself would cause it to refuse to move forward. This technique is known as jading and is still used by horse trainers today. There were also pleasant smelling things that were used to make a horse move forward or calm down. If the substance was an oil it could be wiped on the trainer's forehead, they would then stand in front of the animal and the smell would draw it towards them. .... There were also pleasant smelling and inviting materials, such as sweets, that the horseman could keep in their pocket in order to calm, attract, and subdue a crazed horse. (link)
Even more fascinating, this article points to the author of "The Horse Whisperer," Nicholas Evans, being influenced by his own research into the Horseman's Word when writing his famous book. It says:
Evans [said] he first heard about the phenomenon from a blacksmith in Devon. "I had always wanted to write a book about an ancient subject set in the contemporary word [sic] and this seemed perfect," he said.
"My research took me to Scotland where I read academic papers from Stirling University about a secret Scottish sect called the Horseman’s Word.
"It was such a fascinating subject, and I got more and more obsessed with it."
Evans discovered some of the key phrases used by members of the Horseman’s Word to control their horses, such as "Two as one". The phrase was said to hold magic qualities but was also meant to sum up the idea that horse and man worked together as a unit rather than as master and beast. (link, originally from the publication Scotland on Sunday, December 29, 2002)
The article goes on to discuss an interview with the last known living member of the Horseman's Word, Billy Rennie, who describes many things he was taught as a child and how he was initiated.