Rainbow was the World's Slowest Pony until an accident made her the Fastest Mare Alive! With help from her new friend Fluttershy, Rainbow became The Flash. But she not alone, Gilda has also been in that accident became her reverse. Now the two, that were once friends, fight for city. One to rule and one to save. Not tie to my The Dash story. Rename the Story Title.

Rainbow Dash as The Flash
Other as Super Villains and Allies

A different version of Rainbow Dash becoming The Flash. Might follow the comics a bit more then the show.