I ride in OR and I usually take just a cantle bag even in the winter. I have a lightweight emergency raincoat, rain pants, extra waterproof gloves, a windbreaker (holdover from summer), halter and leadrope. Sometimes my lunch is in there. Of course, if it is pouring down rain to begin with, I don't ride!
Any other stuff (lighter, tp, snacks, leatherman, wild rag, camera, phone, hat cover, chap stick, ibuprofen) is in my horn bags. Water bottle hangs off the saddle.
The trail is the thing....Louis L'Amour