UPDATE: The contest is now over. Thank you to those who participated. According to Equestrian history, it is said that Princess Luna would give the moon for the night right? Well, in the Season 2 episode, Hearths Warming Eve, the play stated that the unicorns used their magic to give the moon for the night? Looks like Lauren Faust needs to take logic class again. -.-
P.S. If someone gives me a good theory OR answer, they will recieve a gift on their talk page. So keep in mind that Equestrian history isn't always accurate NOR correct.
How about you guys try to find something wrong in Equestrian history? Let me know in the comments of what's wrong!
UPDATE: All I could say is that you must remember that Princess Celestia and Luna was around the time even before Equestria was founded. This accounts during Equestria's establishing, Discord's reign, and King Sombra's reign.
IMPORTANT NOTICE: I will say my theory and give the winner the gift after at least more than 18 comments.
DOUBLE UPDATE: There are now 18 comments, I will not tell the winner until my next blog post.
Here is my theory:
Princess Celestia and Luna must've shown the 3 tribes their power, and told them of how they were alicorns, then they must've realized that they were more powerful rulers, and wise ones. So they basically became the rulers of Equestria WAY before Discord and King Sombra.