In the thread.

Yep, passed a hunter on a road right outside of Riverwood who didn't want to talk to me. It was very close to the river.

couriers often ride horses.

They do, just walk around on the roads, you'll see one

I've not seen any NPC's ride horses, and more surprisingly is on certain quests where you can follow the quest giver, even on long trips they run on foot all the way, with one female quest giver we ran near halfway across the map.

Passed a Noble just moments ago riding a horse. He was an ass.