She is Loyalty. She will be the last one standing by her friends, in peril, torture and death.

This is what everyone believes. This is what Twilight and the other Spirits of Harmony, Rainbow's friends, believe. And they took it entirely for granted. After the Mare-do-well incident, Rainbow Dash's Loyalty began to falter. She still felt the terrible sorrow of being deserted by everypony, to be forgotten in the shadow of a greater hero.

Over time sorrow grew to anger. As the powerful bond of friendship crumbles as Rainbow begins to drift away, The others discover just how powerful the Element of Loyalty is, and how its absence might have caused the downfall of the greatest heroes Equestria has ever known.

Okay. So Mare-Do-Well has been unmasked, Rainbow Dash learns her lesson, and everyone lives happily ever after, right?

Not exactly.

The Mane 6 return to Town Square to explain everything to the townsfolk....and they learn that their plan was......significantly flawed.

Twilight hoped that mare-do-well would teach Rainbow to be humble. Instead, she leaves; just leaves. When they find her later, she lashes out at them. When even the Princess fails to mend fences between them, she along with the rest of the mane 6 attempt drastic action to fix their friendship. An accident during this leaves to Rainbow heavily wounded and with few supplies, in the hostile and merciless lands outside Equestria. Can she rebuild her life? Maybe make something more? Or will she fall, starving, thirsty, wounded, far from home, broken, and abandoned?

Whatever may happen, bear witness.....to Rainbows Lonesome Road.

A real hero doesn't brag?

Just because she likes to talk about how awesome she is means Rainbow isn't a hero?

After her friends revealed themselves to her, Rainbow Dash reacts to Applejack's words in a completely different way than what's expected, and things only go downhill from there. Tears will be shed, friendships will be broken, and knives will be twisted as everything that can go wrong WILL.

Alternate ending to The Mysterious Mare Do Well.

Nopony has seen Rainbow Dash since the argument about Mare Do Well at Sugercube Corner, over a week ago. Too much time has passed without an attempt at fame, so Fluttershy goes to Rainbow Dash’s house to talk to her. Instead of finding a prismatic Pegasus, all Fluttershy finds is a letter.
A letter of apologies, a letter of dreams, a letter of fears.
But also a letter of farewell.

You all saw how the episode ended: Rainbow Dash unmasks Pinkie-Do-Well, the others show up, she learns her lesson, a letter is written, and everything ends up hunky-dory. Well, if I know Dash (and my fellow bronies), I'd bet a group of us thought that this was not at all characteristic of our favorite speed-demon. I have my own interpretation of that day's events, respectfully submitted below for your reading pleasure.

Having lost her one greatest fan to the usurper, Rainbow Dash is brought to a new low. How did she lose it all? Even her five greatest friends seemed to have abandoned her for Mare-Do-Well. And then Scootaloo... No. She would not let this happen. She would not lose everypony she cared about to some mask-wearing intruder. Even if she had to go beyond the limits of legality, morality, or even sanity, she would bring Mare-Do-Well down.

Loyalty is a two-way street.