I got a lot, so you might want to get comfortable.
Fleur De Lis, Rarity, and Trixie can all speak French. However, they can’t actually speak to each other very well because Fleur speaks “French” French (I don’t know if it has a specific name), Rarity speaks Canadian French (and a little French French), and Trixie speaks “Creole” French (yes, I stole that from the “Lunaverse” series of stories).
Trixie grew up in an orphanage, but was adopted by her uncles, Flim and Flam, where they taught her how to manipulate ponies. She has since then left them for good, and kinda regrets the influence they had on her.
Fluttershy’s father was an Earth Pony, who was killed and eaten by a dragon, utterly traumatizing her.
Fluttershy’s mother comes from a long line of expert flyers, whose parents (Fluttershy’s maternal grandparents) strongly disapproved of her marrying an Earth Pony. Fluttershy’s mother was incredibly relieved that Fluttershy was born a Pegasus, as this satisfied her parents’ desires to continue the family flight tradition (Fluttershy’s great-great-grandfather was one of the original Wonderbolts)
The reason Fluttershy can fly as fast as even Rainbow Dash (if only during times of great stress) is because of sheer natural ability, as opposed to Rainbow Dash’s combination of natural talent and constant training. She’d much rather stay near the ground, though, despite her ace-Pegasus roots.
Queen Chrysalis isn’t the only Changeling Queen, just the one with the largest number of Changelings under her command. Changelings function a bit like bees (having a hive, a central queen figure), but Chrysalis isn’t a constant egg-layer like a queen bee (she still lays the occasional clutch of eggs, however).
Any unicorn can learn how to teleport. The reason most unicorns don’t is because it’s just ridiculously dangerous and requires a fair amount of magical skill. Twilight, having studied the proper spell back to front can teleport like it’s nothing.
All ponies can use their tails to grab things, but Earth Ponies have a special knack for it, being able to control their tails to a much finer degree. This may be natural, or it may simply be because Pegasi and Unicorns don’t bother to learn (having “wing-hands” and telekinesis, respectively)
Twilight and Cadance, due to not being born alicorns (like Celestia and Luna), are mortal.
Octavia is related to Pinkie, but not very closely (like, first cousin).
Derpy has a “glass eye” (a prosthetic eyeball). Sometimes she wears an eyepatch.