The mare in my Avatar does not sweat and never has since she was born. I have tried Once AC, the Finish Line brand product, dark ale beer, electrolytes. You name it, we have tried it! Nothing has worked. I had her Thyroid tested a few years ago and it was VERY low, the lowest reading my vet had ever gotten on a Thyroid. She was put on Thyro-L and is now up to a normal Thyroid reading when we draw blood. The Thyro-L did help. She now sweats between her back legs and on her belly a little bit. Still not a normal sweater and only sweats a little bit when it is VERY hot in these 2 locations, but the Thyro-L did help. We do fans on hot days, hose her off, etc to keep her cool.
Hope you have better luck than I did with the sweating products. They seem to work great for some and not at all for others!
Jen