It could be for a number of reasons, but I think vetilation is KEY! I took my girl to a show in a beautiful, large, brand new float last year and the lady towing the float closed up all the windows/holes etc because it was raining. When we opened the float it was so STEAMY and the poor horses were drippinggg!! If theres a breeze coming into the float I think any horse would be more comfortable, maybe you could make sure all the vetilation points are open