I jammed my middle finger a few days ago, and it really hurt. It still does, actually. It changed how I moved, how I used my hands, and even what things I chose to do or not do. I’m a person. I can talk. Had I not complained about it, no one would ever have known I had any pain as I was still able to do all the things I normally do, but with subtle compensations, some hesitation, and in some cases a lot more time. You’d never know to look at it that anything was wrong - no obvious swelling or bruising, just a subtle change in posture that made it look like I was vaguely flipping off the world as I went about my day.
Your vet may have been practicing veterinary medicine since God was a boy, and went to vet school four times just for fun, but there is no way they can completely rule out pain. And yet your horse could be hesitant, resistant, non-compliant, or a whole host of derogatory labels, when really there is some intractable pain somewhere in their body that no modern medicine or diagnostic device is able to detect. And, since pain happens in the brain and not in the tissues, there could be no physical evidence to go on. The official diagnosis becomes “it’s just behavioral,” which is an unfortunate catch-all that not only puts the responsibility on the horse, it doesn’t contain any information about how to help. Furthermore, the fact that there’s abnormal behavior at all is a clue that something is actually wrong - behavior ALWAYS has a cause, and fight/flight behavior is often caused by, you guessed it, pain.
Unfortunately, this leaves us in a really messy place. But we were already there. There are so rarely definitive answers or checklists or protocols that tell you exactly what to do. This is life, and it’s so much bigger and more complex than a YouTube video telling you how to get your horse to do <insert desired behavior here>. We have to be observant, and considerate, and we may have to sacrifice our goals for the short term, or possibly forever, for the sake of the horse’s comfort. They didn’t sign up for this, and it’s our responsibility to make sure it’s as easy for them as we can possibly make it. Otherwise, we’re just riding on our entitlement.