The Duchy of Lancaster is an ancient body responsible for managing an investment portfolio of land, property and financial investments. It also undertakes various administrative duties associated with the area of the historical County Palatine of Lancaster (today, primarily Lancashire, Greater Manchester and Merseyside).
Click on the following link to view the 'History of the County Palatine' page on the Duchy of Lancaster website.
http://www.duchyoflancaster.com/about-the-duchy/history/county-palatine/
Unlike the seal of the Duchy of Lancaster, which shows the Monarch enthroned, the seal of the County Palatine shows the monarch mounted on a horse in the manner of a medieval knight - even Queen Victoria.