So I have a friend that I went to a show with last weekend. It was upwards of 90 degrees F. We did a pleasure division (3 flat classes) and after that her horse was breathing VERY rapidly. The classes were very short (because of the heat) and he is used to working more than that. The only sweat on him was under his saddle. My horse, however, was drenched. Clearly it was hot enough. We got him back to our popup tent set up next to our trailer and he took a drink and she took his tack off and sponged him down. His breathing came down and he chilled out for a while. She showed him in some over fences classes later that day and he seemed alright, but still didn't break a sweat.
I've never seen a horse breathe so fast in my life, not even ones that have just ran a race. And now that I think about it, I rode him a couple times for her just at the barn to give him some exercise and on any normal day that my horse would sweat up a storm, her horse never really did sweat much at all.
Also, he is a 17 year old TB and weighs around 1250lbs (overweight, clearly lol)
Also, we thought about dehydration, but he did pee on the trailer on the way to the show and didn't seem to want to drink a lot.
In the last two years he has gone from being a hard keeper to getting fat off of air. She didn't seemed to worried about him, but I'm curious to know what might be going on with him. A metabolic disorder perhaps?