Here is a quick brief History on the development of Current Nepal Flag,
If you need to know how Nepal flag has been developed, you need to go back to 5000 years ago, during the time of Mahabharat and kirat king Yalember,
Also This flag is based on Hindusim, In Hindusim, most of the symbol are encoded/ painted in traingle. IF you see "OM" sign, Hindusim used to consider "OM" sign in traingle base as holy.
Now lets go back to Mahabharat and king Yalember period. You can easily see Kirati flag in two traingle with bow and arrow. Also in Hindusim, political power were divided into two group which is "Surya bansi" and "Chandra bansi".
Pandavas were Surya bansi, and For your information. Ram was Chandra bansi, that is why Ram was called as ramchandra. They believe Surya and Chandra as a power.
If you see the flag of Arjun , just refer to GITA, you easily see the flag of Arjun, Its two traingle with "OM" sign.
And See the flag of Kirati period, its a two traingle with bow and arrow.
see link for GITA and Arjun with Flag.
See link for Kirati Flag:
History tells that King Yalember and Mahabharat was in same time frame,
Kirati rules over 2100 years, after that area was ruled by licchavi , thakuris, mallas then Shah dynasty,
There is no exact evidence i found what was the flag during Licchavi, thakuris but in mallas period, you see two traingle flag with surya on the top and chandra on the lower traingle in all 3 malla durbar of kathmandu, lalitpur and bhaktapur.
see the link below for the image of main gate at bhaktapur durbar square.
The reason behind Surya on the upper triangle and chandra at the lower triangle during mallas period is, Malla rules were Surya bansi.
Now you can imagine why current flag has chandra on the upper triangle and surya at the lower triangle.
Yes, King Prithivi Narayan Shah belogs to Chandra bansi. So after he unified the 22 and 24 rajya along with the kathmandu valley, he was the one who modified the Current flag of Nepal with blue broader, red background with chandra on the upper triangle and surya in lower triangle, That represent, Chandra bansi claimed victory over Surya bansi, as shah dynasty own over malla dynasty,
If you are Nepali do not just bull shit. The flag represent the history for over 1000 years ago.