The three tails and three stars represent the three tribes from which Equestria was formed; no specific symbols are used to avoid giving any tribe prominence over the others.
The sun-moon symbol represents the princesses and the Equestrian identity, which serves as the uniting bond between the three tribes (or something like that).
The indented edge mirrors the rays of the sun, and is mostly there because it looked kinda cool.
This design takes inspiration from:
Flag of Gondor
Flag of the Most Serene Republic of Venice
Flag of the Turkic Council
Flag of the Mamluk Sultanate
Imperial Standard of the Qing Emperor
Flag of Anyuan Miners’ Strike
““Arms of the Qing Dynasty[](https://upload.wikimedia.org/wikipedia/commons/thumb/7/77/Arms_of_the_Qing_Dynasty_28fictitious29.svg/700px-Arms_of_the_Qing_Dynasty_28fictitious29.svg.png)
Chinese fishermen’s flags