In today's Rainbow Dash review, you learned that she was based on Lauren Faust's childhood Firefly toy. Specifically, this Firefly toy:
Here's what she looked like in the cartoon:
It's still hard to get the connection between that old pony and the modern one, right? Things become a bit clearer when you see Faust's rendition of Firefly, done in the Friendship is Magic style:
Okay, suddenly that makes a lot more sense! It's all about the attitude.