Rainbowdash is a giant six-legged horse of Shadowfel origin. She originally served as steed to the similarly giant Facecrusher, but following his defeat by The Party alongside the other Riders, Little One adopted her. She now serves as his leadership cohort.
When LittleOne made his armour, he also made a set of barding for Rainbowdash with a platform on her back to carry the party over long distances. More recently, Draven has crafted her a helm of intelligence, making her technically sapient, if not all that bright by humanoid standards.