Rainbow Dash- I thought she would be born with a rare trait of having 6 colors in her tail and not her mane, which is white with the primary colors. She would grow her tail out so when she zooms she leaves behind her iconic rainbow trail. She would keep her mane short to keep it out of her face.

Fluttershy- She seems like the type of pony to keep her mane and tail in a bun, and so it doesn't get her her way too much for her to help her animal friends. You can't see her cutie mark, but its a Bear paw with butterflies around it.

AppleJack- With her I wanted to add the handkerchief as a normal cowboy/girl thing. Then I put her mane and tail in braids instead of her low ponytail as it would be easier to work with.