This is not Fluttershy. This is actually Fluttershy's ancestor. Who happened to look a lot like Fluttershy. This is not just any mountain. This is the mountains not far from were Ponyville would one day be founded.
FS: "I have failed in my quest. Foul beast, I have doth chased you all over Equestria only to be denyed my victory." She stares up at the blocked cave entrance near the top of the mountain. "Sleep and let not sweetness enter your dreams. Dream of the day when my descendant will one day return to finish you. You will feel the sting of Shy family. For it is our honor to defeat you...... Dragon."
Many years pass as the world changed and her words came true. As Fluttershy climbed the same mountain (all be against her will) to face the same dragon her ancestor faced. Only the sting was not of a sword but the powerful words that would change the dragon instead of slaying it.