She is a light blue earth pony with a pink, orange, yellow, and green mane and a pink, blue, and purple tail. She features a cutie mark of a rainbow stretching between two clouds. In 2009, during My Little Pony's "Generation 3.5" mass redesign, she was shown with a large ponytail in her mane.
Rainbow Dash is described as stylish and fashionable. My Little Pony calls her "The Glamour Girl of Ponyville." She is a kindhearted pony and is very interested in fashion and she also has a tendency to use say the word 'Darling' very often.
Note: Nothing is wrong here. I am aware that as of 2010 she was redesigned as an athletic pony. It isn't right, however, that all the definitions of her leave out this chunk of history, so I wanted there to be a definition on THIS Rainbow Dash as well. Though a lot of people will be tempted to downvote this because it's a side of the character you don't like, It's important that this be defined as well.