The song is from season 9 episode 7 of MLP:FiM.
Main song: https://youtu.be/ET--RKu9sr0 | ♫ Lyrics for the song:
( You can support the show here: https://itunes.apple.com/us/tv-season/my-little-pony-friendship-is-magic-season-9/id1458376743 )
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[Rarity]

You've mastered so many pony ways
And grown a lot in the past few days
You've shown a turn that has earned our praise

[Yona]

And now I fit right...

[Rarity and Yona]

...in

[Rarity, Fluttershy, Rainbow Dash, Applejack, and Pinkie Pie]

You fit right in!

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"My Little Pony" is owned by Hasbro,
"MLP: Friendship is Magic" is created by Lauren Faust.