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
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.
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.
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.
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.
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.