It was all a mistake.
A portal opened up during their Wonderbolts training exercise and Rainbow Dash fell through, saving a teammate in the process... But at a cost. Now stranded in an Equestria that is all but foreign to her, Rainbow Dash must figure out why exactly she is there and what she needs to do to get back.
Yet when something happens that Rainbow could never have been prepared for, she decides that enough is enough. Stricken with guilt and compelled to do what's right, Rainbow finds herself stuck between a rock and a hard place. With her name already meaning nothing so far in the past, she decides to call herself one that she still viewed as the epitome of awesomeness and nobility in all of history...
A Wonderbolt.
Captain America: Pony Style! - Wonder Edition! Find All The Answers In This Fantastic New Addition!
If you haven't already, read the prelude one-shot Daring Do and the Frozen Wonder. I wrote it so the beginning made a little more sense.
If that makes sense.