Rarity uses the term "Mane Six" in "Ponies of the moment", a term used by the fans and some official materials but never spoken onscreen during Friendship Is Magic.
Cathy Weseluck doesn't voice Spike in Pony Life because it would ruin her memories of voicing Spike in My Little Pony: Friendship Is Magic (2010). John De Lancie has also opted out of voicing Discord in Pony Life, and Spike and Discord are both non-pony characters.
The second spin-off series based on Friendship is Magic, with the first being Equestria Girls.
Often referred to as Generation 4.5 by fans.