The one thing I always consider is that if it is extremely hot and I need that much water, then I really need to limit the duration and speed of our rides.
I buy western saddle bags and attach a strap to them so they clip on my English saddle in the front. The bag’s weight sits in front of the saddle on the horse’s neck. No rubs or complaints from the horse. The only thing i need to pay attention to is keeping the bag balanced so it stays centered. If you drink all the water on one side of the bag, it will slip sideways.
I typically carry four 32 oz bottles in the summer ( 1 gallon total). The Ozark trail bottles you buy at walmart. I want enough water to dump on me or the horse if it gets too hot out. And that is for an hour to hour and a half ride in the summer.
We rode 10 miles today and I drank all 6 cups (and 2 cups before I left the house). I should have brought more water then that as I drank everything I brought with me. And it wasn’t even hot today, not by Florida standards.
I try to bring more water then I think I might need. You never know if you might get stranded. Good to pack food, a cellphone and wire cutters.