Human Headcanons GO:

Twilight and Shining Armor:

I actually have several headcanons..... 1, is that Twilight's family are Indian, with Asian [Korean] blood, since Twilight Velvet is mixed.
The Korean part doesn't really show through Shining, but it does through Twilight Sparkle.

2. Twilight is a black American.

3. She's a full-blown East Asian, probably Korean.

And finally, 4. one where's she's a mix of the two above.

The Pie Family:

1. Mixed Race. Caribbean-American [Jamaican-Bahamian mix] on their Dad's side and Swedish on their Mom's. For most of the sisters, it's obvious that they're biracial, with the exception of Maud, who can easily be mistaken as a full-blown PoC.

2. Pinkie's Native Hawaiian, with African blood.

3. Pinkie Pie's biracial [Native American/African]

The Apple Family:
Ohh boy..
Applejack and her immediate family are African-American, of the Southerner variation.
Babs is part black, part white-Italian, and probably light-skinned.
Braeburn is partly Scot-Irish American.

Rainbow Dash:
1. Latina. Mom's Cuban, Dad's Domincian Republic, and one of her Grandparents were Mexican.

2. Rainbow Dash is biracial [part Latina, part Afro-Hispanic], probably with the same principles above.

Rarity and Sweetie Belle:
Rarity and Sweetie Belle are multi-ethinic: Part Sudanese-British, part Egyptian Arab. The former on their Mom's side, the latter on their Dad's. Rarity has a nice phyisical mix of all of her backgrounds, but Sweetie Belle could be Egyptian passing if it wasn't for her (or his, in keeping with my headcanon) curly hair. Oh, and they have a bit of Italian-French blood, which is fine with Rarity, since she will always adore French culture.

Fluttershy:
My most prominent one is that she's mostly Apsaalooke Native American, but partly Japanese-Chinese and French-Canadian. They all show.

Another one is that she's Japanese.

Or Persian.