Exactly what the title says.
To those who don't know what "Core 7" is, it was a time when Hasbro decided to only produce 7 ponies: Pinkie Pie, Rainbow Dash, Scootaloo, Cheerilee, Toola-Roola, Sweetie-Belle, and Star Song.
This decision made fans of the series facepalm due to the fact all they were doing was reproducing the same ponies with new accessories. Soon after the Core 7, they released G3.5 and we know how that turned out.
Luckily Lauren came along, essentially slapped Hasbro in the face and made what can essentially be called "G1 with G3 colors".. AKA, G4.
I'm a fan of G3 Rainbow Dash.. Before the Core 7. They changed her personality for the revision, and took away her British accent.
Edit: My reaction has changed since I made this. I like Core 7 - though not to the extent as pre-Core 7.
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