Again, everyone is entitled to their opinions and choices, but the results can't be debated.
I wish people would check out my links there's a lot of science and facts mixed in there ;)
This is one that specifically answers your question about why, even though they're a prey animal, they're still easily trained with CT. Our Equine Clicker Training Methods | Cascade Institute of Equestrian Studies
Also in the clicker training thread I keep referring everyone to, on the third page it clearly explains why CT works for all animals, not just predators.
Excellent question and one that I've also seen come up a lot.
Bottom line, every living thing needs energy and usually devotes the majority of its life working to obtain it by consuming food in some way/shape/form. Horses are no exception. It is a simple fact of life: horses need food to survive and so they work for it - whether they're in the wild and searching for grazing grounds or domesticated and chasing the rest of the herd off their flake of hay. Since horses work so hard to obtain their food simply to stay alive, it's an easy thing to exploit as a reinforcer/motivator in clicker training, especially if you use something like enjoy
eating and don't get all the time.
Where predators are concerned, I'd actually be more worried about using food with them than I would non-predators. Most predators, like dogs, are fine being given treats. However, there are a few (snakes come to mind) that like their prey to be alive and lose interest in meals that aren't moving. If you were to clicker train an animal such as this, you'd have to find something that motivates them. If squirming meals were the only thing that motivated them to work, you'd have to use that as your reinforcer!
Herbivores, on the other hand, actually need more food than predators to function and that is why they are ALWAYS eating. Plants actually do not contain a whole lot of nutrition, so herbivores eat a lot, poop out most of it, and so must continue eating more. In contrast a predator, like a lion, can get all their nutrients from one meal and some can go weeks without eating.
Which brings us to the issue of being full. Any animal using treats in training does risk getting full and losing motivation to eat (like when you have a HUGE meal and don't even wan to look at dessert! Rare, I know, but it does happen lol). You don't need a starved animal, but right after feeding time probably isn't the best time to try clicker training, either. Generally, if you work with your horse any other time than after feeding time, they should be decently motivated to work for treats since they have such high energy (food) demands. Horses allowed free-choice hay and grass are usually ok since they're getting a slow and steady food intake (as opposed to stuffing themselves once or twice a day) and should still want food when you're working with them since they're working for food all day anyway. However, you'll need to pay attention to your horse and get to know them to find out when his optimal training time will be based on the desire for food (or whatever your reward is) and any other factors that affect motivation and attention.
All of this applies to all animals for the same reasons - here are a few examples of "unlikely" animals working with clicker training, none of which are predators:
Goldfish (I don't think they're predators, and even if they are, how incredible that CT is so simple and elegant it can be used with virtually any animal!): https://www.youtube.com/watch?v=D4kPZ25IMn0
And, not gonna lie, this is my favorite one I found for so many reasons and I LOVE these camels! (And for us CT junkies, check out the targeting, the "stand" game aka "stand on your mat", and the camel/trainer reaction to when the camel asks for food!) https://www.youtube.com/watch?v=ShI6WYlSdz8
I can tell you with my own horse her desire for the C+T has helped her overcome her flight instinct. My mare who was terrified of life itself has overcome all of her fears, she's confident and eager to work - with the use of CT. I can tell you I tried -R with her and when she was afraid the level of violence it took to get her to give in was beyond what I was willing to do to an animal I claimed to love. When she was afraid of going through her gate I lunged, yielded, whipped, and backed this horse for several hours before she gave me one step out the gate. I needed to hurt
her to the point she thought I was going to kill her if she didn't go through something she was sure was terrifying, but she still
was sure that the scary thing she didn't know was way more scary than me. I'm not willing to hurt my animal at that level. Straight up bribery didn't work either, just leaving the gate of her dry lot open into a field of grass got her to reach out her head, but never take that step. But after a week of CT confidence building and targetting I had her walking in and out the gate and all around my property. Now she goes wherever without questions.
If you aren't willing ot escalate to the level of violence it takes to FORCE a horse to do what you ask, then you need to make them want
to do what you ask. Those are your options. Some horses may only take some quiet persuasion to get them to obey, while others will fight tooth and nail, it's up to you to decide what level of aggression is more than you're willing to instill on your horse and whether or not it will actually work in the end, are you strong enough to hurt your horse enough to make
them obey? If you physically or emotionally aren't strong enough maybe you should look into alternatives. CT is the other option, it makes the horse want
to work for you, because finally there's something in it for them.
You don't have to use it but please
learn about it before you trash it. Don't just scan my posts and slam it down, read the articles, learn the science.
ETA: If a goldfish can be clicker trained a horse can! xD