George B. McClellan designed a lightweight military saddle in 1859. It became standard issue in the US Army and was exported to several other countries. Now a McClellan saddle is almost synonymous with a "military" saddle - the design is still used by ceremonial units in the US Army. It's similar to saddles used in Mexico.
McClellan saddles are still available for civilian use and are sometimes used by endurance riders.
Image source: SimonATL via Wikimedia Commons.