A Canon-Incompatible Concept regarding Exceptions to Standard Pony Tribal Categorization

There is a concept for interpretation of the show, a sort of... potential headcanon, which I have contemplated (but never personally held) that I solves a number of inconsistencies and confusing elements in the show, but which is a bit too far from what show canon has told us to be considered perfectly consistent with existing canon; however, it's close enough that it wouldn't be entirely inconceivable for an author of the show to suddenly introduce it without completely overwriting show canon.

As a brief refresher of the relevant information:¹

• Ponies can be divided into three tribes; Unicorns, Pegasi, and Earth Ponies

  • Unicorns have a horn, with which they can directly use magic
  • Pegasi have wings, and can thus fly (they can also walk on clouds)
  • Earth ponies have a "good strong heart", but I'm pretty sure that's really just said because it's better than "earth ponies are lame and don't have any special powers."
• There are, additionally, Alicorns, which are essentially the combination of the three (unicorn horns and pegasus wings, plus they're sometimes really big, ostensibly because Earth Ponies). Alicornhood is generally associated with royalty, and is generally earned rather than inherited (Journal of the Two Sisters may or may not contradict this, but Princess Flurry Heart canonically does).
• Despite there officially being only 5 Alicorn princesses (Celestia, Luna, Cadence, Twilight Sparkle, Flurry Heart⁴), several winged unicorns have appeared in the background, generally classified as animation errors.
• Despite being royalty (as should be obvious due to being an Alicorn), there are a lot of occasions (particularly in Manehattan, if I remember correctly) where Twilight is not treated as one would expect people to treat royalty.

I have a concept that resolves this, but which is a bit too far outside of show canon for me to accept as a consistent headcanon: "winged unicorns" can exist, which are not Alicorns, but exhibit both wings and a horn. "False Alicorns" would make up a tiny percentage of the population – perhaps around 0.5% – so that their existence would be well-known and not jarring, but so that they would be little more than an asterisk in books about the three pony tribes. Anypony who doesn't treat Twilight as royalty could conceivably be assuming she's just a false alicorn (Ponies don't watch enough TV for her face to be instantly-recognizable nationwide, so while they may be aware of a Princess Twilight, they wouldn't make the connection). Animation errors where a background pony exhibits wings and a horn could be reclassified as False Alicorns.

Unfortunately, this doesn't solve every problem, and raises a few of its own:

• Rarity appearing as an Alicorn in Pinkie Pride can't be written off as a False Alicorn, since we already know she's a unicorn (the same applies to any other animation errors with known-tribe ponies).
• Flurry Heart being immediately identified as an Alicorn, along with the claim that "the birth of an Alicorn is something Equestria has never seen", wouldn't make sense if this headcanon were applied.
• Occasionally, ponies do recognize Twilight as royalty by her wings, like in The Cutie Map.
• The fact that the show has entirely contradicted this on countless occasions (either in the description of Alicorns or in the enumeration of pony tribes) is a bit of an issue,⁵ but could always be written off as "technicalities being omitted for simplicity".

In conclusion: I find this to be an interesting concept that could have been introduced to the show somewhere in the first few seasons, but by now (between seasons 7 and 8), too much contradictory canon has been established for the writers to enter it comfortably, and accepting it as a headcanon is rather difficult. However, it solves enough problems that I kinda wish Faust, in all Her wisdom, had introduced it in the beginning.

¹ Whoo! Exposition dump in nonfiction²³ writing!
² I mean, it's about fiction, but it's not written as fiction but... agh! What do you even call this sort of writing‽
³ Don't just love footnotes within footnotes? Also, footnotes on other footnotes?
⁴ Unrelated, show-compatible headcanon: Flurry Heart isn't the same kind of princess.
⁵ Ah, fandoms. "But that's entirely wrong." "Yes, that is bit of an issue."
⁸ That wasn't an exponent.