In my club we cannot ride in the morning or at night as each lesson has its own schedule. So when it's really hot, we go in the wood and just walk :) we also shower the horses after the lesson
And funny fact, we put sunscreen on white horses (limb and nose)
Shower is the best way to struggle against the heat to me!
The essential point is to ride before sunrise or after sunset. Or not ride at all if possible. I always try to make my horse drink a lot and shower the legs, hearth girth and neck.
When it is really hot, I like to put a fan in the stable of poor horses. I ride early in the morning or late in the night when the temperature is lower. And I often shower them