Add on: if it's overheating, it may be your best course of action to stable him during the day with a box fan on him.
It might be worth trying a respiratory supplement, and if that's doesn't help, I'd call your vet. Horses can sort of have asthma, and they'll be able to help you determine an appropriate treatment plan.
My mums horse gets this due to the Pollen, if there are lots of trees or just grass where you it could be due to that. The pollen count has been really high over the summer and now the wind has brought back a load of dust which doesn’t help! She has to have a nebuliser (it’s like an asthma pump) for a few days every time she gets bad... hope your pony is ok ❤️🙂
I try hosing him and it helps some.
If you have any thoughts on what else to do or what it may be besides overheating let me know please.
btw this horse is a 40yr old welsh pony, darker color (i’d say almost a flaxen love chestnut)
i have a video of him breathing heavy the app just does not allow videos to be shared.