When the Windigos attacked, the ponies had to find a way to put an end to the distrust and anger that fed the frozen fiends. The unicorns found a way. No more earth ponies, no more pegasi, no more problem.
A thousand years later, Princess Twilight Sparkle thinks that her ancestors may have made a mistake. Fortunately, she knows a way to test her hypothesis. She names that way 'Rainbow Dash.'