Nepal is the only country without a rectangle or a quadrilateral flag. The flag of Nepal has two pennons or pennant ( this is the correct term used to describe a long trinagular flag).
If one looks at the history of the South Asian countries and our own region (Udaipur, Chitral,Dir, the Mughal and Sikh empires), a triangular flag and unusual shapes were common because each King or kingdom or princely state had their own flags until the British, Portuguese etc descended. Once the British empire established slowly the flags of the states were phased out and the rectangular shape was adopted.
But Nepal has since ancient times had a flag that resembles two triangle placed one on another. The oldest flag is said to likely be designed by a King of the Rana dynasty many centuries ago. It is often attributed that the King of Nepal - Prithvi Narayan Shah in 1774 AD changed the then existing flag with moon coming above the sun. The moon stood for the Moon dynasty kings and the sun stood for the Sun dynasty kings. It also had faces similar to humans to represent the sun and moon gods. This was changed in 1962, wherein the faces were removed.
The red colour is that of the offical flower of Nepal, the rhododendron. The moon implies the royal home and the sun implies the Rana dynasty. The blue border stands for peace and harmony.
If someone wants to draw the Nepal flag exactly, then refer to Page 235-237 of this pdf about the Nepal constitution (http://www.wipo.int/edocs/lexdocs/laws/en/np/np029en.pdf) which gives detailed instructions for the same.