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Impressions before reading: The description sounds a little high-action for the SoL tag, but then I'm hardly one to speak when it comes to misusing that tag (what do you do when your story isn't an adventure, isn't dark, doesn't have romance, and isn't comic?). If this is going to be about civilizing a child, then a lot is going to rest on making that child an interesting, sympathetic character, while still keeping it from becoming just an adult in a child's body--not necessarily an easy task, but one that can be very rewarding for readers. Here's hoping that's what we get!
Zero-ish spoiler summary: While out in the Everfree Forest, Fluttershy comes across a young pegasus who's lived outside of civilization for years, ostracized for his slit-pupiled eyes... and his knowledge of an ancient martial arts technique which gives him almost unimaginable prowess.
Thoughts after reading: If you type some variation on "what makes a bad OC" into your search engine, you'll get plenty of advice--oddly enough, mostly pony-specific. At a glance, "shipping him/her with a main character," "ridiculous superpowers," "special and unique physical characteristics that don't actually make him/her ugly," "canon-breaking abilities," and "mysterious backstory for the sake of drama" are a few of the more common ones which could be applied to this story's protagonist. However, these aren't really how you make a bad OC; lots of stories have been based on characters with one or more of these traits, who were nevertheless strong, interesting, and well-designed for the story they were in. That list is a list of symptoms, not of causes.
So, the reason I take issue with Dawn Lightwing isn't because he displays those traits; it's because of the problem from which those traits all spring, to whit: he's a fundamentally dull character. From his Lt. Data-like dialogue to his almost total emotional passivity to the fact that essentially everything he does in the entire story is reactionary (rather than borne of any sort of personal interest or initiative), Lightwing is less a character than an otherwise unmemorable vehicle for all of those symptoms. There are some hints of actual personality toward the very end of the story, but 100,000 words is simply too long a wait to introduce any personal characteristics. For the rest of the fic, he's just an unrealistically unchildlike (not simply toughened beyond his years; he talks, thinks, and behaves like an emotionally stunted adult, not a child (emotionally stunted or otherwise)) cypher, and the fact that his defining characteristics are things like "can fly better than Rainbow Dash, is more magical than Twilight Sparkle, and can turn aside Applejack's kicks without effort" makes him fundamentally unlikable. Those are things you could get away with as elements of a well-rounded character, but when they ARE his character, that's when you have a problem.
Frankly, the titular foal himself wasn't even my biggest problem with the story; that would be the subplot in the early going about a serial attempted rapist, which appears to exist solely for the sake of letting Lightwing prove how badass he is (he quite literally disappears from the story, nary to be mentioned again, as soon as Dawn finishes beating him up). Leaving aside any question of how tasteful it is to use rape that way in a story in the first place, the rendition here includes some awfully questionable characterization of both the rapist and the victims--I can't go into too many details without spoilers, but the speed with which Fluttershy's friends were pushing her back onto the dating scene literally days after she was violently sexually assaulted appalled me.
Although the fic is well-edited in terms of punctuation, word use is a constant weakness. Phrases full of overwrought descriptions and needlessly obtuse writing, such as "[His voice] also carried a hoarseness that suggested that the mechanisms that facilitated its usage had not been exercised for quite some time" are common. There's also a tendency to pile on extraneous details in the narration, but to be fair, these were at least occasionally interesting in their own right.
But the biggest problem here, which both Lightwing's lack of realism and the rape subplot's beyond-questionable construction are facets of, is that there seems to be little, if any, connection between character actions and characterization. Applejack has an absolute, sycophantic faith Celestia's divine ineffability, despite the fact that she never once shows any unusual obsequience to her in the show; Lightwing speaks with a wisdom and (conversational) maturity unmatched by any other character in the fic, despite having spent the last year or two of his short life totally isolated from anypony at all; Scootaloo's parents have somehow managed to be happily married for years before the fic begins, despite the fact that [SPOILERS]. It's difficult to imagine how the ponies could possibly have reached the positions they're in at the start of the story given their actions and attitudes, and so their plights and conflicts feel contrived and artificial.
The best thing I can say about Foal of the Forest is that the ending gives me hope that the multiple sequels are better than this is. But for most of my reading, I was either being made profoundly uncomfortable by unrealistic and transparently narratively convenient trauma, wondering whether some over-the-top event or reaction was intended ironically, or both.
Recommendation: Readers who are prepared to invest the protagonist with personality themselves, and who aren't going to think too hard about how and why characters act the way they do or what those actions imply, might get some enjoyment out of this, if they're into DBZ-style martial arts (the "proving badassery via ridiculously one-sided combat" bits, that is, not the "charging up for forty episodes" stuff). For the general reader, though, I don't recommend this.
Next time: Wonderbolt, by WovenWord
...But first, we're going old-school! Next week is Retro Week, as I go back and grab three 6-star fics that I haven't reviewed yet, starting with...
Dresden Gets Schooled, by Chengar Qordath