Context: TCB happens and Humanity is forced off of Earth, after reaching another planet we rebuild our civilization. But due to a variety of disasters we kind of had to start over from scratch, so no perfect digital records. You know the lost colony trope.

How would future humanity try to explain or rationalize the very bizzare series of events that transpired on Earth? For instance maybe the commonly accepted theory was that it was some mutating virus that emerged due to climate change and ravaged Earth. Many may find the idea of actual magic horses invading Earth to be too implausible, kinda like creationism.

7586255
Really simple: we were invaded by interdimensional aliens that could eat basically anything green and thought that us eating meat was the cusp of barbarity, and that us not automatically loving and worshipping their goddess-empress made us heretics, so they made a bioweapon to make our brains into psycho-vegans like them. Thankfully, a handful of us managed to escape. And now we look up into the sky with fear, hating any wolves in sheep skin that may want to assimilate us.

Come to think about it, I guess the new human society would be very liberal internally but very xenophobic externally. The ponies were intolerant of us humans being different from them, so we would be better than that by choosing to love and tolerate each other in our differences and diversity. Due to population growth nobody would be allowed not to reproduce, but assisted reproduction could help jump the hurdle for gay men and lesbian women.

\
7586402
So instead of accepting the reality that it was a natural disaster of two universe crashing together and one of those universes contained a radiation that was deadly to humans but was part of the natural metabolism of that other universes natives... they needed a villain, a bad guy, someone they could blame it all on. Same way conspiracies theories form. If there's someone you can blame, there's someone to overcome, someone to defeat, and that will make things better.

On a similar subject, people often get prayer backwards, it's not about influencing God, it's about aligning our will with God's.

7586416
The "historical truth" and the actual truth would depend on what Conversion Bureau universe you're basing yourself on. The history I'm proposing would work on any where the conversion serum got improved away from a large-dose drink (because drinking 23 ounces is a lot more contact than breathing a gulp of a gas or a drop to your skin) and where there was a PER movement. Or any where you simply had an expanding barrier, even without a PER or an improved potion. I would be the actual truth in anything ever written by Chatoyance (if any humans managed to build an Orion Engine in Australia and then landed and slowly colonized Mars), or in a pony victory in the Negotiations-verse, but wouldn't work at all in a Yellowstone or an Option Gamma (the one where you could convert into a gryphon).

7586402
7586404
In that sense you would see a lot of demonization and exaggeration of their evil, perhaps things would take a religious bent. You know the fic "and hell followed with them", ponies would probably look like horrible demons in popular culture, and Celestia is literally satan.

On the second note I figured that humanity would be more or less that already since it would be in the future where humanity is overall more tolerant. But either way I don't think we'd regress aside from nationalism eventually being a thing again.

7586416
History is written by the winners is bullshit, but I'd imagine that there would be more HLF around to control the narrative. It depends on how passive Celestia was in all this
7586490
Chato Celestia struck me as being passive, but considering how hostile humanity is written it would be going against the grain if humanity initiated hostilities and the writer wasn't off-putting (but cordial)

Addendum: I remembered several stories where humanity starts it and was generally off putting. (Author tends to be okay people)