State and Naval Ensign of Siam (now Thailand) in 1817-1855. It's a red rectangular flag with a white elephant in the Sudarshana chakra. Adopted by King Rama II (Buddha Loetla Naphalai) of Siam. This image is an adaptation of Image:Flag of Thailand (1782).svg and Image:Flag of Thailand 1855.svg. 

Please note that there is/was no specification design for this flag.
Own work
This  vector image was created with Inkscape .