Unicorns Are Magical, by wille179
Zero-ish spoiler summary: In an alternate universe where unicorns are governed by the same morality as the Unseelie, five ponies are visited by a lavender mare who can create wonders--or terrors.
A few thoughts: I really wanted to like this story--the idea of completely recasting the entire race of unicorns as malign fey seems ripe with possibilities--and on some fronts, it delivered: without turning Twilight into a simple evil genie (i.e. granting everypony's wishes in the most unhelpful way possible), it made her feel alien and menacing. Beyond that, the reverse-chronological order of the five characters' scenes is used to good effect. Where this fell short for me was in connecting to the characters it was nominally supposed to be about; Twilight is one thing, but I had trouble connecting several of the other girls to what I expected of them, in terms of personality and desires.
Recommendation: If the idea in the summary intrigues you, this is worth a try. If you're looking for something that expands on the show, or the ponies that inhabit it, than something that's basically original fiction, it might be less to your interests, though.
The Monster of the Hive, by Charon the Chronicler
Zero-ish spoiler summary: After being cast out from Canterlot, Chrysalis decides to use the surfeit of love energy she still has to create a super-being, far more powerful than any ordinary drone.
A few thoughts: This is mostly a vehicle for some meme jokes, and while it's somewhat better-written than stories of its type usually are, there's still nothing here that elevates it above "a vehicle for some meme jokes." That said, the brief visit to the minotaur settlement-slash-4chan's /sp/ board was self-aware enough, in its silliness, that I found it remarkably bearable. Besides, FiM minotaurs are intrinsically funny, yaknow?
Recommendation: If you cringed at the phrase "vehicle for meme jokes," you know perfectly well not to bother with this story. But if, for whatever reason, you're looking for something that will let you turn off your brain and laugh at basic referential humor for a couple thousand words, you could certainly do worse.
Daring Do and the Curse of Ahuizotl, by Horizon
Zero-ish spoiler summary: Daring Do recovers the fabled Amulet of Tlaloc.
A few thoughts: You know, I thought I'd reviewed this story long ago, but I guess I only talked about it in the comment thread from the writeoff for which this was originally written. There's a bit of a good news/bad news situation, trying to write a review for this story: on the plus side, it's an excellent horror story with a chilling reveal (some readers missed the reveal; if you do, go back and re-read the last sentence, or just look in the story comments). On the downside, I'm pretty much at a loss for what to say about it that will be useful, but won't spoil the twist. Um... it's a really well-done horror story, at barely 1000 words? That seems pretty vague, but it's better than nothing.
Recommendation: Look, it's short enough that if you have any curiosity about what the titular curse actually is, you owe it to yourself to read the story and find out. This is an easy recommendation for fans of darker stories in general, and of horror specifically.