Written by Bad Horse, this is the story of what happens to a (late adolescent) Fluttershy, a few years before My Little Pony: Friendship Is Magic's first season, when she attempts to go out and make friends with other ponies. Available here.
Tropes in this story include:
Adult Fear: Fluttershy doesn't wind up getting abducted by demons or attacked by monsters. Instead she winds up getting seduced, used and abandoned in a one-night stand, something all too likely to happen to a beautiful unchaperoned innocent in a singles bar, even in a culture as nice as Equestria's.
A Lesson Learned Too Well: "Don't trust strangers." Fluttershy becomes terrifed of strangers. A few years later when Twilight Sparkle meets her, Fluttershy is well on the way to becoming the Equestrian equivalent of a crazy cat lady.
Break the Cutie: Fluttershy's story arc, made all the worse because she blames what happened to her on her own personal inadequacy as a lover, or possibly as a pony in general, rather than on the callousness of Nosey aka Smiles.
But Liquor Is Quicker: Fluttershy is very unused to alcohol, and she gets drunk. This is partially Nosey's fault, partially Fluttershy's own naivete about the likely consequences of drinking too much. It's about half of the reason why she's willing to sleep with Nosey on a single night's acquaintance.
The Casanova: "Nosey" aka "Smiles," who seduces and uses Fluttershy, then doesn't even want her address. In context, Fluttershy is one of the most beautiful and loveable mares in that universe; his reaction implies that for him predation was the whole point of the exercise.
Chivalrous Pervert: The first stallion she meets, who kindly tells her that she's a nice kid and really should just go home and think about whether she actually wants to be doing something like this. She should have listened to him.
Downer Ending: Not only has Fluttershy lost her virginity to a jerk, not only did he show by leaving without even talking to her, throwing away her address, and tossing aside her freakin' flowerpot which she had given him as a going-away present, but Fluttershy doesn't even get the point (that she should be more careful about whom she sexually trusts). Instead, she decides that she is so incomprehensibly and irredeemably inferior to all other mares that she should just avoid all equine contact. Which is where we see her at the start of the series. Yeah, things get better for her in the series, but that must have been a lonely time for her, alleviated only by her occasional meetings with Rainbow Dash and Rarity.
Flowers Of Romance: Specifically invoked by Fluttershy when she gives Nosey the chrysanthemums — when she discovers that he callously tossed them away the moment he was out of her sight, this is what causes her to realize that he didn't even particularly like her, let alone love her.
Hate Sink: Based on the fan comments, Nosey/Smiley. Some of them go so far as to want to abuse him personally. Probably because he's a Karma Houdini (see below).
Heroic BSOD: It isn't shown on-scene, but it's fairly obvious that Fluttershy completely breaks down at the end of the story. Keep in mind that this is Fluttershy who is ignoring her animals.
She gently picked up the fallen flowers in her mouth. They needed to be replanted as quickly as possible. So she turned around and galloped all the way back to her cottage. But when she got there, she dropped the flowers in the yard, went inside, and shut and locked the door behind her. The hens kept milling around the yard for another half hour, waiting for seed to appear, before they gave up and returned to roost in the warmth of their coop.
Horrible Judge of Character: Fluttershy, who doesn't grasp what actually happened until she sees the discarded flowers. And even after that she blames herself.
The Ingenue: Fluttershy has obviously never even gone out on a friendly date with a stallion before.
Karma Houdini: After taking advantage of one of the kindest, most innocent mares in Equestria Nosey/Smiley trots off into the sunset, having had his way with her and wreaked enormous emotional damage upon her in the process, and suffers absolutely no harm in consequence. This may be why he seems to be such a Hate Sink of a character.
Virginity Makes You Stupid: Fluttershy displays extreme naivete in trusting Nosey, whose background (a literal traveling salesman) and smarmy ways make his dishonorable intentions fairly obvious to the reader. Justified, in that Fluttershy is extremely naive, not so much because she's a virgin as because she has little to no normal social experience, and her concept of relations between the sexes has largely been formed by sentimental romance novels. In consequence she loses her virg9nity, but it doesn't make her noticeably better at dealing with stallions — instead it leaves her utterly-terrified of Ponies in general for years to come. Thanks, Nosey.
Wrong Genre Savvy: Fluttershy's concept of relations between the sexes comes from romance fiction. Sorry, 'Shy. This is realistic tragedy you're in.