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DISCLAIMERS:
This is NOT MY STORY. I simply contribute some material for it.
Continuity is managed by /mlp/RGRE
This document serves only to highlight core continuity moments for newcomers to the prompt and to explain my works' place in it; there's more to it than written here, and continuity can and occasionally does shift over time.
Multiple links under entries are to be viewed in listed order.
There has been a lot of material thrown around this prompt, often branching in different directions, so below I'm only including 'major' contributions, which I'm considering any purpose-made artwork and any greentext sequences requiring multiple posts. Some are written by me, and some are not.

CONCEPT:
Anon got Isekai'd straight into Celestia's womb as part of a plot by Discord to distract Celestia in a plausibly deniable manner. His mind's that of a human, his brain and body is that of an alicorn foal, and it makes everything really fucking difficult. Because of the ensuing developmental delays, a lot of ponies assume he's mentally challenged in some way. Prompt follows him as he grows up.

CHRONOLOGY:
rgre prompts pastebin How to get it? rgre prompts pastebin
Post that started it all is here; https://desuarchive.org/mlp/thread/34446693/#34450697
Discord wants to tie Celestia down with a bit of immaculate conception, but can't actually makes souls, so he reels in the first he sees leave a body Out There, Some Where.

While Anon's still very young, Nightmare Moon returns years early for some unexplained reason. She goes to confront Celestia but instead finds her nephew, born while she was imprisoned. Turns out NMM really wanted a foal. Redeemed on the spot.
Depicted in;
https://i.imgur.com/PVlR2lV.png
https://i.imgur.com/5bMj2YT.png
https://pastebin.com/6eJu7jrb

Luna eventually manages to dreamwalk into his head, and learns the truth about his past. Now understanding her nephew's problems, she becomes ferociously protective of him, but keeps his secret.
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Depicted in; https://pastebin.com/YrLuFTNa

Young Anonicorn was quite active in palace life, even if he attempted to remain unobtrusive and out of the way. He touched the lives of many palace personnel in his tiny benevolent way, and in a few cases, resolved a couple hitherto-unknown diplomatic issues. His reach and presence spoke far more than his size and demeanor would lead one to believe, rippling throughout the Canterlot court and beyond.
(presented in no given order, and the description is a bit of a gloss-over; these greens are good enough they each deserve their own section but they all cover a similar time period with similar subjects. Do read them all!)
https://pastebin.com/K8njUxCi
https://pastebin.com/xbvSjhdy
https://pastebin.com/FREvWjEA
https://pastebin.com/da1fibja
https://i.imgur.com/G1Iskb1.png

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Anon grew up with Twilight, Rarity, and Blueblood. Unlike Anon, who's merely autistic-adjacent due to his unique conception, Rarity is actually autistic.
Justification; https://s2.desu-usergeneratedcontent.xyz/mlp/image/1571/26/1571263506728.png
Twilight is pretty dismissive of him, and generally treats him like an object of curiosity more than a real pony, because she can't into friendship yet and Anon can't do magic despite being an alicorn. Shining Armor occasionally has to run interference between his sister and his lord's son.
Rarity and he get along like two peas in a pod, because Rarity gives no fucks about his lack of interaction skills, being utterly blind to such things in the first place, and Anon likes someone who treats him like she treats everyone else, even if she treats everyone else kind of weird.
Blueblood grows up as his minder, of sorts, helping him pick up some social graces. He's also a total fucking bro despite his attitude towards the low-borns, and the two distant cousins would do anything for each other.
Rarity ends up moving away during older childhood, while Anon is stuck with Twilight longer because she becomes Celestia's student.

Raven Inkwell helps take care of him while he grows (maybe as a private tutor?), as Celestia's right wing mare. She's often with doting-mother Celestia and young Anon. Anon might develop a childhood crush on Raven
E.G.; https://i.imgur.com/ND4pB1f.png

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As Anon grows, he acclimates to his body and Equestria, leaving behind his developmental challenges and becoming a pony who is mostly normal, save the hangups that came around as a consequence of how ponies treated him during his younger years. He's quiet and has a massive insecurity complex, coupled with an entirely understandable imposter syndrome problem. Still hasn't really gotten the hang of magic.

At a royal event/gala, Twilight and Rarity (along with the rest of the Mane 6) attend. This is Anon's first exposure to his two childhood companions since they moved away. Rarity, being socially-blind mare she is, bungles the reunion, which Blueblood tries to help Anon with. Twilight senses a FRIENDSHIP LESSON OPPORTUNITY and makes things worse.

Between this botched reunion and Twilight's ascension, Anon gets a severe inferiority complex thinking Twilight's going to replace him in his mother's eyes. This eventually leads to him running away from home, which crushes Celestia. Cadance eventually finds and rescues him from Glimmer's Cutie Commune, and takes him to the Crystal Empire, where Shining decides to try to teach him some magic.
Depicted in;
https://pastebin.com/zdhGJWLA
https://pastebin.com/CYabsTTr
After this all settles down Anon returns home.

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Eventually, Anon starts expressing interest in mares, later than usual but it does arrive. Luna, being overprotective and invested in The Old Ways, decides any potential suitor must face her in combat to earn the right to court him. Rarity returns to his life, and confesses that she never had a heart for any other, then manages to defeat his aunt for the honor.
Depicted in;
https://pastebin.com/dhtELhCs
https://i.imgur.com/oEpI8r1.png
https://i.imgur.com/DN8oi5T.png

Rarity's courtship pays off, rough as it is with her social difficulties. She receives the honor of being the lead mare of a herd of (currently) one. Celestia and Luna grant Anon and Rarity the ruins of their old castle and a workcrew to rebuild it, so they can be closer to Ponyville.
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