Butterfield Overland Mail route. This route actually predates the Pony Express and generally is less known. I found a sign about it at a park in Benson and have never heard of it before. This was actually a stage route and not a purely horse route. The data was taken from a map I found on the Library of Congress website and various other maps from the NPS. I made it pass through the main cities and adjusted it to modern highways where it made sense to me. The route is nowhere near exact but gives a close idea of the route. Some sites like Benson, AZ and Fort Bowie in AZ have actual historical displays.
Citations
https://parkplanning.nps.gov/projectHome.cfm?projectID=33568
Lloyd, James T, and Rae Smith. Lloyd's American railroad map of the United States, showing the three proposed roads and the overland mail route to the Pacific. [New York, 1859] Map. https://www.loc.gov/item/gm70005361/.