Five Score, Divided by Four is a My Little Pony: Friendship Is MagicFanfic by TwistedSpectrum. It follows the adventures of Dave, and later others, who after his 25th birthday wakes up and finds Rainbow Dash's Cutie Mark on his thigh, which is only the start of a long chain of changes that lead to him becoming the mare herself. And he's not the only one exchanging hands for hooves.Dave and others must find out why this is happening to them, all while dealing with the difficulties and consequences of turning into ponies in a world built for humans. Sufficed to say, Hilarity Ensues.Has a NSFW version at derpy.me
Apocalyptic Log: Fiona's diary has the feel of this even though we know what happened to her, and she's found still alive in the following chapter.
Discord is subjected to this fate after his defeat: to live 6,250,000 (Five score to the power of four) years on Earth as a normal horse.
Batman Gambit: Dash and Shining's rescue of their captured comrades plays into a variant of this as they provide an anonymous tip about the below-mentioned 'terrorist bombing', knowing the cops, FBI and various other agencies would rush in on their friends' captors, allowing them to escape in the confusion.
Call Forward: News of an explosion in Seattle that was reported as a terrorist bombing turns out to be Team Canada using an explosion as a distraction to escape after robbing a bookstore of an MLP series bible.
Another example is when a little girl calls Dave "Rainbow Dash" and tells him really her to beat Discord. It turns out she was right.
Did You Just Punch Out Cthulhu?: Dave/Rainbow Dash does this literally to Discord. At mach 1 via a Sonic Rainboom. The blow wounds him so badly he's left helpless as he's sentenced to his fate. Justified as the Cutie Mark Crusader and friends used the Elements to strip him of his immortality, but still quite the feat.
Hoist by His Own Petard: Discord throws a seemingly helpless Rainbow Dash very far into the air, hoping to kill her. She gets a Heroic Second Wind and by throwing her, he put her in perfect position to come back and give him a Sonic Rainboom powered blow that defeats him.
Ironic Echo: Only noticible in the NSFW version. Dave shares Fiona's bed platonically one night. The next morning, both Jack and Evan say the same thing: "Jeez, Dash, you really sleep around huh?”, neither realizing she is, but not with Fiona.
Karmic Transformation: Discord is subjected to this, being transformed into a normal horse for one million years.
Metamorphosis: The transformations of the main characters into ponies qualifies, taking place over a period of days and being presumably irreversible, as it's not a case of humans turning into ponies, but more accurately a case of ponies that were turned into humans turning back into ponies.
Oh, Crap: Discord has one when he realizes that the Elements have rendered him mortal after Sweetie Belle cuts off several of his talons.
He has another when he's been badly injured by Dave/Rainbow Dash to the point he can barely move, let alone defend himself, and Applejack and Big Macintosh decide to take some well deserved retribution on him.
Twilight: The hour has come. So, um, according to most books and movies I’ve seen, at this time I should give you all a rousing speech before the big battle before us. (Raises hoof.) Make sure you all are brave. Historically, that is the trait that is usually beneficial at a time like this. (Lowers hoof.)
Later in the same chapter, Shining Armor gives one before leading the Equestrians into battle, and it is glorious.
Shining: EVERYONE! LISTEN UP! As many of you know, I am Shining Armor, brother of Princess Twilight and former Captain of Royal Guard. Ponies... I welcome you to Equestria.
Twilight: What’s he doing?
Rainbow Dash: He’s leading.
Shining: I know some of you have looked outside and are beginning to doubt why we are here. You are asking yourselves why have we come to this wasteland? Why are we risking our life? Why should we fight? But the answer to that question is clear as day to me: This is our home! We all knew what would be waiting for us here, and on behalf of all ponies in our kingdom, I thank you for your courage. I know most of you are not fighters, and I know we lack any real experience fighting the sort of things that lay before us. We are students, secretaries, office workers, gamers, and just brothers and sisters. None of us have been trained to deal with the threat we will face here, but we will face it. I know you are scared right now, but you shouldn’t be! Look around you, look at the hundreds, nay, thousands of ponies who were brave enough to come here and face the peril! You are among them! You are just as strong as they are! Think of what we can do, together! My fellow ponies, today you will stand among thousands of the bravest individuals any of us have ever seen. Think of our combined strength! Think of our combined rage! The fury that we have been waiting to unleash on this day, on this day twenty five years in the making! My fellow ponies, if you think you are scared of the enemy, you must imagine how scared he is of us! Imagine the sheer terror gripping their black hearts as they realize we have arrived on their doorstep! Imagine the terror they must feel as they realize that before them is an army of the bravest, strangest, most furious individuals this world has ever seen! An army that has been waiting twenty five years for revenge! An army here to take back what is rightfully theirs! My fellow ponies, we are that army! Imagine the terror in our enemies when they realize today is the day they will have to face us! Today is the day we reclaim all that we lost! My ponies, today we take back that which was stolen from us, that which we lost over five score divided by four years ago! We will get revenge on that great traitor, that great coward who backstabbed our beloved Princess and our kingdom! Now, I don’t know what Discord will throw at us, but I do know that we will not run away any longer! We will hold this ground! This land! Right here! This land, right in the ruins of our beloved town! Not only does this land connect back to our friends back on Earth, but this land is the most important land in Equestria! You know why? Because this is land that Discord no longer controls! WE HAVE TAKEN IT FROM HIM! And from this spot, we will take back everything else he has stolen from us! He won’t take a single inch of this land back from us! He has taken enough! Your old friends, your family, your past TWENTY FIVE YEARS! Ponies! If today in battle you or your friends start to feel fear... If you want to just run away and let Discord’s forces to take this land from you... you must think back to everything Discord has already taken from you! Think what he has taken from all of us! Are you willing to let Discord take even more!?”
Shining: WE WILL NOT GIVE HIM A SINGLE INCH! NO MORE WILL HE JUST TAKE WHAT HE WANTS! NO MORE WILL HE TAKE WHAT IS RIGHTFULLY OURS! HIS TYRANNY ENDS TODAY! TODAY WILL BE A DAY REMEMBERED BY ALL PONYKIND AS THE THE DAY WE STRUCK BACK. TODAY WILL BE THE DAY WE GET REVENGE FOR EVERYTHING WE LOST. FOR OUR FRIENDS. FOR OUR FAMILIES. FOR THOSE TWENTY FIVE YEARS. FOR EQUESTRIA!”
Surprise Pregnancy: Dave fears one of these after a nightmare, only to be reassured she isn't by Fiona.
Triang Relations: Rather complicated versions considering what's happening to them. Evan - Dave - Jack in a type 7 Evan and Jack (Twin brothers and transforming into Big Mac and Applejack respectively) are both sleeping with Dave (Rainbow Dash) without the other knowing, though Dave doesn't want to cause friction between the two of them, and Tom - Rachael - Harry in a very complicated sort of 8 Tom (Rarity) and Rachael (Shining Armor) had a crush on each other before the transformations began, meanwhile Harry (Pinkie) is just interested in a sexual relationship in the new body with the only stallion in town. Unlike the other group, however, both Rarity and Pinkie are aware of each other's feelings and don't mind sharing Shining. However, if the show is to believed, Shining is married to Cadence, and none of them are sure how to handle that, if Cadence is even in this world.
In the last chapter, Cadance and Shining wound up splitting things off due to being completely different, thus taking her out of the equation before she could even make the love Triangle a Love Square.
Wham Episode: Chapter 11. Dave realizes they are and always have been the ponies they're transforming into. Jack avoids a potential Fate Worse Than Death. Dave locates Fiona, who has turned into Fluttershy.