Jargon is a terribly difficult problem for any writer. Using too much loses the reader that isn't conversant with it, but not using enough, especially in the dialog of expert characters, can ruin them. They lack depth & aren't believable. Using just enough, yet making it understandable to the uninitiated, is an art. One of the best is Janny Wurts. Her book "To Ride Hell's Chasm" revolves around horses, their frailties & strengths, yet those with no experience with them can still understand the issues. She uses just the proper amount, with short explanations believably aimed at less knowledgeable characters, & turned this problem into an asset. We subtly learned volumes about the characters & world in an interesting way.
Every sport or specialized endeavor has its own jargon, a necessary way for other practitioners to communicate quickly & clearly with each other. Horsemen have a number of different jargons depending on their area of origin, what sort of horse they have, & how they use them. Those of us that ride English hunters may share some of our specialized vocabulary with those that use drafts for hauling wagons, but not as much as you might think. The work & issues are completely different. Even among riding horses there can be a lot of differences.
Mixing jargon can be horribly jarring. If a cowboy put a headcollar on a horse, it would jar me right out of the story unless there was a good explanation for it. The British use 'headcollars' while most in the U.S. use 'halters', even if they ride English & use many other terms in common. Cowboys 'saddle up' while English riders 'tack up', although both put 'tack' on horses. Small, enclosed fields are 'corrals' out west, but we call them 'paddocks'. English riders use a 'girth', western riders use a 'cinch', but both pass over the girth area of the horse. Out West, their jargon is full of Spanish terms.
As a general rule, pick your lingo & stick to it, but every general rule has an exception. Correct terminology can override common usage & a mix of styles, types, or people will mix the jargon. While we only ride English, one of our neighbors has Walkers & Saddlebreds, several others ride Western, & one is a vet. When we all get together the conversations can quickly become a confusing mess of different lingo. We'll try to use the other's jargon to be polite or make points in their terms. Sometimes we don't get it right. It can be funny or frustrating.
I've had trouble understanding my wife when she tries to correct my riding. Many of her corrections sound remarkably like English, but they just don't make sense. For instance, she told me "my hip was too open" over a jump the other day & seemed to think that explained the issue. I hadn't a clue what she was talking about. I finally figured out that I was standing up a little too much which opened the angle between my thigh & belly too far. So why didn't she say so? Why is the floor of a boat called a deck or a sole? (What's the difference?)
Horses & Horse Information has a pretty good glossary of English equine riding terminology, although Discover Horses might be better. For other types & disciplines, you'll have to do some research & there can be a fair amount. Something as simple as a bit is actually incredibly complex. Take a look at the Horse & Saddle Shop's rundown on their bits. It's hard to believe, but my wife has a trunk full & still mourns the loss of one that she hasn't been able to replace for over a decade. Golfers are as fussy about their equipment. I think it is the curse of the expert & most heroes in fantasy novels should be experts. They trust their lives & the fate of the world to their horse all the time.
Horace T. Ponii