A ponified version of the anime "Rokka No Yuusha".

According to legend, when beasts awaken from Tartarus and turn Equestria into a nightmare, six heroes will be chosen to save their great land. These heroes are known as the "Braves of the Six Elements". Among those chosen is Rainbow Dash, a pegasus who proclaims herself "The Strongest Mare in the World". However, when the Elements finally meet, there are seven of them, rather than six.

Note: Although this is based on the anime "Rokka no Yuusha", you don't necessarily have to have watched the anime in order to understand the story. However, I highly recommend you give it a look if you're into adventure/fantasy.