My own headcanon for MLP:FiM

[HorsesandMOARGaloar 300]

(Warning: much of it is influenced by fan works)

 

Biology:

 

An Equestrian is always born with a distinct, usually-nice-smelling odor: for instance, Rarity's breath smells of marshmallows.

 

Society and History:

 

Even if there may be no temple in Ponyville or Canterlot, the Equestrians might worship and pray to Celestia, "Empress Mother" Omnia (a.k.a. Fausticorn), Luna, or any one of them. Newly-transformed alicorns NEVER count.

 

Omnia's first creation was the Tree of Harmony.

 

As creator of the planet Equestria and its Universe, Omnia also created Celestia and Luna, and lived with them in a castle somewhere in what will become the Everfree Forest, where they spent their foalhood. Upon coming of age, they were presented to the newly-united pony tribes who then chose them to be their rulers.

 

Whenever an Equestrian ("Pony") speaks any language (usually English, though you can teach it any language you like), its words are always understandable to any horse..

 

Some Equestrians know Latin, but they regard it as a formal and a magical language. It might have been written with glyphs.

 

Even though they are naturally inclined against it (except for Pegasi, who usually enjoy it), Equestrians are capable of waging war and using (or inventing) deadly weapons, even if these weapons do not have magic.

 

Executions are very rare in Equestria: they are reserved for the worst of all monsters.

 

(Semi-) Fanon:

 

Lyra loves hands and people, but she will be frightened of people who chop off hands, even if they're done for her.

 

Queen Cheeselegs (and most Changelings) does not like fluffy objects.

 

Fluffle Puff's fluff is a genetic disorder.

 

Applebloom is Pinkie Pie's baking apprentice.

 

It is rumored that the pegasus Surprise is actually from the sturdy "G1" Universe, brought towards Equestria by "Omnia's will".

 

EqG (Oh, the horror of it all!):

 

The "Humans"/"Pony People" in "Equestria Girls" are not true Humans: genetic tests proved that their DNA has lesser Human elements. About their "cutie marks'" thing, we will not know.

 

Flash Sentry (the pegasus) can be capable of an awful fit of rage: he might even attempt to pursue you to torment and kill you, simply for slandering him!

 

The EqG Universe, accessible only via only one magic mirror, is: either Celly's stimulation of the wonders (and horrors) of Human life (with the Ponies as Humans); or a certain unicorn's dangerous experiment to see how those "legendary" Humans lived, risking a horrible punishment from "Queen Mother" Lauren. This can also be why it was sealed deep into the Crystal Empire's royal palace.

 

Hub-promos/-related:

 

If this was to be counted as "true", Dan and Blythe may be among the few Humans who have successfully visited Equestria (?), and survived as intact Humans.

 

Contrary to popular belief, Equestrians cannot be killed with GAK, nor do they get frightened of it (unless it's used as a flood or a poison, which is a more sensible threat).

 

Miscellany:

 

If Krastos the Glue-Maker does exist in the Everfree Forest, he might as well be the last/only native (?) Human in Equestria. (Same goes to Chester the Cynical Adult, who may be his best friend).

 

A Houyhnhnm (a race of intelligent horses from "Gulliver's Travels") must not speak of the "First Mother" to an Equestrian; otherwise, he or she will end up in a ridiculously furious debate about Omnia being or not being the "First Mother", and offense that the Houyhnhnm vehemently rejects the Pony's teachings of "love, tolerance, harmony, and friendship"; the tenets of the native Equestrian religion.

 

Before she created Equestria and its Universe, "Empress Mother" Omnia was taught and trained by a mysterious teacher, who created five earlier universes (known to us as "G1, G1.5, G2, G3, and G3.5"). While the other four "rotted away into sweet, sugary oblivion", only "G1" survived decently, but at a lower quality than Lauren's Universe, "G4".

 

Hugh Jelly might be Derpy Hooves' brother, so it might make him Ditzy Doo's uncle.