I have ridden in 110 with a 75% humidity... that was... yeah. Toasty. Our saving grace is we were in the woods on trails and the wind off Lake Texoma, as we got close to the water, was refreshing.
Water, water, water. Keep you AND your pony cool with water. For riding and for yard work - I work all summer through the heat, all day long - keep your HEAD soaked, keep those cool towels you have around your neck - they will cool the blood supply to your brain.
Sports drinks like gatorade are optional - but I've noticed Gatorade of today is NOT the salty/minerally/barely sweet drink it was when I was in HS and college. Had one the other day - it tasted syrupy, almost as sweet as actual Hawaiian Fruit Punch. They've packed a LOT of sugar in it now. So. To keep your electrolytes up, take a break, have a pickle juice shot or eat one of those huge dill pickles like they sell at ball games. Keep lemons and salt on hand, watermelon, cantaloupe, avocados, bananas, nectarines. Have those and a bottle of water for a snack when you start to feel light headed - but don't push yourself too far. A snack like that will come right back up if you're too hot and scarf it down too fast (BTDT)
Give your horse a hose down too, but be careful to not soak him with too cold water while he's hot. I let mine have a cool down period before they get a bath.
Take your horse swimming if you can - I don't necessarily mean like you see in videos and movies where you're both in over your heads - and if you DO try that!!!!! BE SURE you use split reins and no tie down.
What I mean though is if you have a lake or a pond you know is safe to wade in, lead them out into belly deep water with a halter and lead rope, no saddle. Let them play but be careful you don't stick in mud and fall down and get pawed - they won't be aiming for you, they'll be stomping the water to get cooled off.... I had one, Sarge, and he would motorboat - stick is nose in the water and blow bubbles, then lay on his side and swim around, find the shallows, wallow in the mud. Then he was done and happy. Trigger runs circles around me. Gina paws water up on her sides and back, and she's DONE. Dee yew enn ee DUN. Superman stands there in total bliss and likes to have it scooped onto his back. Each is different, and your horse will be too. Just use an abundance of caution if you go with the swimming/wading option and let your horse choose to follow you into the water.
"We are here to laugh at the odds and live our lives so well that death will tremble to take us."