Rainbow Dash is a female pegasus and one of the main characters in My Little Pony Friendship is Magic. She is responsible for maintaining the weather and clearing the skies in Ponyville. She represents the element of loyalty.
In Mari0, while she herself does not appear, her mane and ear is the 33rd hat that the Player can wear. The Player's hat will also be set to it after preforming a Sonic Rainboom once you enable the ability to do so in the Cheats menu..
In Mari0: SE, Rainbow Dash is a playable character. Maurice streamed him coding "flight" and actually playing as her on Twitch. MM102 also made a Rainbow Dash character, which was later included in beta 9, although her flight ability was removed.