As early as the 1780s, the Post Office Department contracted with stagecoach owners to carry mail along designated on post roads. Private carriers, using stages and wagons, transported mail across the trans-Mississippi west in the 19th century. But horses did not always pull mail vehicles—sometimes they were the vehicles themselves, including the horses used by the famous, privately-owned Pony Express service from 1860-1861.
Horse-pulled wagons were also familiar sites in American cities. Screen wagons were used to carry mailbags between railroad stations and post offices. The screened sides and rear locks insured the mail’s security in traffic and worked so well that when trucks began to replace wagons, many were built with screened sides.
When horses were unavailable, or unable to perform that duty, other animals were brought in, depending on the region. While mules continue to carry mail in the Grand Canyon, camels were used briefly in the southwest, but proved poor mail carriers. Some rural carriers have even strapped oxen to their wagons for making their daily rounds. But the most commonly used replacement for horses were dogs, used to pull mail sleds across snowy, frozen areas in northern U.S. states and Alaskan territory.
Nancy A. Pope, National Postal Museum