It's more than just pegasi that participate in the Running of the Leaves. Most of the time it's actually Earth ponies and unicorns. Everypony's welcome to get in on the race, though, you just can't use your wings. They made that a rule after... well, Applejack and Rainbow got kinda competitive one year.

And there are four princesses! So far, anyway. Princess Celestia, Princess Luna, Princess Cadence, and Princess Twilight! Twilight only became a princess recently. It's why she's got wings now. Only princesses are alicorns in Equestria. You've gotta show you're the right one for the job, too! [He looks incredibly proud as he says all this.]