It was sunny when you left but about an hour into your ride a fast moving thunder storm comes right over your head. There are compact waterproof ponchos that are small enough to fit in your shirt pocket. In fact, everything mentioned here will fit into a fanny pack with room to spare. Avoid back packs, they have a tendency to throw your balance off and they can get caught in tree limbs. A long-sleeved shirt and long pants will give you a bit of protection from scrapes and sunburn and it helps to keep the bugs from biting you.
Warm weather brings insects and they can make riding miserable for horse and rider.
So here's what you do. Take a little bug repellent along for the ride. If the bugs start bugging you, apply some repellent to your face and hands. You can also apply some on the horse's face and ears. You'll have a happier horse and a better ride.