One of the more unique wheel arrangements to be applied to a steam locomotive was the 4-8-0 twelve wheeler. The very first 4-8-0 locomotive is believed to have been the "Centipede", a tender locomotive built by Ross Winans in 1855 for the Baltimore and Ohio Railroad in the U.S.A. where it remained in service for nearly twenty years. It appears to have been delivered in a cab-forward type of configuration that was modified to a Camel configuration in 1864. On a Camel locomotive the cab was mounted atop the boiler, unlike the later Camelback locomotive whose cab straddled the boiler and that first appeared around 1877.

The name Mastodon for the 4-8-0 wheel arrangement was derived from the unofficial name of the first 4-8-0 locomotive of the Central Pacific Railroad in the USA, the wood-fired CPR no. 229, which was designed and built in 1882 by the railroad's master mechanic, Andrew Jackson (A.J.) Stevens, at the railroad’s Sacramento works in California.

In general, “Twelve Wheeler” was essentially a freight locomotive and was relatively rare when compared to other wheel arrangements of the time like the 4-4-0 American, 4-6-0 Ten Wheeler, 2-6-0 Mogul, and 2-8-0 Consolidation. The 4-8-0 was meant to be a more powerful replacement for some of aforementioned designs, notably the American and Ten Wheeler, and was primarily manufactured between 1890 and 1900 although some designs were built as late as the 1920s. The type never achieved great popularity, although there were four occasions when a specific 4-8-0 locomotive was considered as the "heaviest and/or most powerful in the world" upon its introduction.

Those locomotives were the no. 20 "Champion", designed by Philip Hoffecker for the Lehigh Valley Railroad in 1880, the no. 229 "Mastodon" of the Central Pacific Railroad in 1882, the G5 class of the Great Northern Railway in 1897, and the no. 640 of the Illinois Central Railroad in 1899. It is noteworthy that the Great Northern G5 had 16-inch-diameter (410 mm) piston valves, as large as the pistons of many locomotives then in service.

Even though, at the time, the wide-firebox 2-8-2 Mikado had much more potential as far as speed is concerned, the Norfolk and Western Railway opted for the class M 4-8-0 for its shorter wheelbase that enabled it to have over 90 percent of the locomotive's weight on the driving wheels, and the four-wheel leading truck for greater stability. Built by Baldwin Locomotive Works from 1906 and nicknamed "Mollies", the class M, class M1 and class M2 became the most numerous American class of 4-8-0.