During the Final dream sequence, Fluttershy briefly turns into "Flutterbat", a fan favorite form of hers from the season 4 episode "Bats" Rainbow Dash and Applejack also turn into their "Power Ponies" forms and Spike becomes his warrior dream form from "A Dog and Pony show"
During the final Dream sequence, Big Mac turns himself into an Alicorn "Princess". His transformation is a reference to both Sailor Moon and an internet meme joking that MA Larson would turn everyone into an Alicorn, even Big Mac.
This is the first time since the Season 4 Season Finale that the rainbow powers have been seen, though it was only during a dream sequence. The Rainbow Powers were widely panned by fans and critics as being too much of a "gimmick" and it is likely due to this that they have not been used since.
The show has never shown any male alicorns until Big Macintosh became an alicorn temporarily in Princess Luna's dream world.
Friendship Report: Luna learned the only way to move forward is accept her past and appreciate how she is now.