Accidentally Correct Writing: G1 featured many, many mares and fillies but only a handful of stallions and colts. In the toys and UK comics, the non-married males live separately from females, while in My Little Pony 'n Friends the males are on a racing trip together. This is actually accurate to real feral horse herds. They consist of females, their foals, and at most one or two full-grown stallions. Stallions leave their herds at a certain age and most live separate from females.
"Unipeg": A formerly popular term for Winged Unicorns until G4. When Friendship is Magic became popular, fans switched to "alicorn" (which actually refers to the material unicorn horns are made of), which ended up Ascended Fanon. "Unipeg" still sometimes pops up when referring to G1-G3. "Pegicorn" was also an accepted term.
"Original Six": The original six G1 ponies: Butterscotch, Blue Belle, Minty, Snuzzle, Cotton Candy, and Blossom.
"Core 7": The seven ponies that became the main characters during late G3 and G3.5: Pinkie Pie, Rainbow Dash, Scootaloo, Sweetie Belle, Cheerilee, Starsong, and Toola Roola.
All the generation lines are technically fan nicknames. G3.5 was never officially anything but a Soft Reboot but it was noticeably different enough to be given its own moniker. Tales is not "G1.5" because only the cartoon adaptation was different, not the toys (which resembled normal G1 toys).
Subverted with a few G3 ponies released through non-standard methods such as cons or mail order. They're referred to by simple monikers like "Egyptian Pony" or "Breast Cancer Awareness Pony".
No Export for You: The biggest case of this is in Japan. Well, it was semi-averted in both G1 and G3 (toys only). To put it simply:
The G1 Toys by Takara didn't sell as well in Japan as in the West, and in a What Could Have Been moment, the baby versions were intended to be released there but were cancelled.
G3 suffered from this too. Takara Tomy re-licensed the series in 2006 and released the toys up to the G3.5 line. It didn't sell well either and Takara Tomy decided to pull the plug on the license in 2012 and shut down the official Japanese website for good. The company had enough with low sales and switched their focus to Transformers: Prime.
The main reason however on ending the franchise in Japan is because it can't keep up with three competing girl-oriented franchises: Pretty Cure, Hello Kitty and surprisingly, Jewelpet (from Takara Tomy's competitor Sega Toys). Even if Friendship is Magic has a decent storyline, characterization and toys that had potential in the Japanese Market, Hasbro and Takara Tomy decided not to release My Little Pony in Japan anymore due to three competing franchises that could crush them.
Not to mention that the fanbase of these three in Japan is as solid as iron compared to My Little Pony overall.
This is now averted — as of January 23 2013, Bushiroad is finally bringing Friendship Is Magic in Japan after a looong 3-year wait. The best thing about this: the cast from Bushiroad's Tantei Opera Milky Holmes franchise are coming back to voice the main characters! Also counts an example of Marth Debuted in "Smash Bros." since this season was the first in the franchise to be dubbed in Japanese.
Strangely, "My Little Pony Tales" suffered from this only when it comes to the toys. Though a US-produced series, the toys were produced everywhere but the US, it seems.
On the subject of toys, any MLP toy packaged inside Kinder Surprise Eggs will never be sold in the United States.
None of the first three generations ever officially made their way into South Korea, just like most Japanese cultural products until between 1998 and 2004. In fact, it wasn't until Generation Four when any My Little Pony toys at all were released there.
Of course, there was a dub for the G1 TV series, but it was treated like a Japanese anime, due to it containing animation from Toei.