Rainbow Dash was a female Pegasus pony who represented the element of Loyalty and was one of the six Guardians of Friendship.
She is presumed dead by the start of the G5 era, having not shown up in other contexts than Sunny Starscout's imaginary playtime at the start of My Little Pony: A New Generation.
Personality
Rainbow Dash was a somewhat aggressive and competitive pegasus. She presented as someone who thought highly of herself, but secretly was very insecure. She cares deeply for her friends and is motivated by them.
Appearance
Rainbow Dash had a sky-blue coat, magenta eyes, and a colorful mane - a red, orange, and yellow forelock with a green, blue, and purple crest. Her tail has all six colors in rainbow order. Her cutie mark was a white cloud with a single bolt of lightning coming out striped red, yellow, and blue.
Concept Art
Earlier G5 ideas featured redesigns of the Generation 4 characters from the cartoon. Rainbow Dash keeps her species and blue coat, but her rainbow elements are played with. Some concept art adds rainbow secondary feathers, and some mess with the order of the colors in her mane. Some concepts give her golden shoes and accessories, giving her a Roman Warrior feel. The manner in which she is drawn conveys a similar personality to that of her G4 edition.
Media
My Little Pony: A New Generation
Rainbow Dash and Fluttershy
Sunny Starscout owns a Rainbow Dash figurine that she plays with. Under the control of Sunny, Rainbow Dash flies in to see her other friends and excitedly proclaims she's "in" for an adventure. She later asks Twilight Sparkle if a foe they're going to face is "awesome". She gets scared and runs away from Rarity (under Sprout's control) attacking the others.
Comics
Rainbow Dash made a cameo appearance in The Storm of Zephyr Heights Issue 2 along with the Wonderbolts as Flash Magnus was on a hologram. She appeared later in issue 3 as a hologram as well as her older version at the end.