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As a result of Rainbow Dash and Applejack's horsing around, I must now conclude that the fastest pony is Derpy Hooves!
(the piano on her car is a reference to this- [link]
Derpy Hooves, Rainbow Dash, Applejack and Pinkie Pie (c)
Ford Mustang GT (2010 & 1968)
Dodge Charger R/T