I don't see how the Rito couldn't have "evolved" somehow from the Zora, although as for why they did doesn't really make since, and Dragon Roost looks more like Death Mt. to me instead of Zora's Fountian. But the fact that they have the same symbol and that the sage preceding Medli was a Zora, and that it's obvious the Koroks "evolved" from the Kokori, seems to imply to me that the Zora, or at least some of them, decided to worship a dragon that could allow them to fly.