What's been teasing through my mind is a story based on the ancient Russian legend of the Firebird, which brings both beauty and misfortune into the lives of whoever may see it, and especially on the one who dares keep one of its glowing feathers. The legend has several variants, generally falling into either a quest to capture the enchanted bird or a quest to find and bring back a princess somehow linked to the bird. Usually, in true fairy-tale fashion, the hero is the underappreciated third son of a king, who succeeds where his more favored brothers have failed. But what if the prince is a princess, the Firebird is far more than it seems, and the fates of many magics hang in the balance? And yes, I think a magical horse or two will play a role, something also seen in some versions of the Firebird legend.
I don't know if it will all come together or if I'm chasing something as elusive as the Firebird itself. But I'm going to give it a try!