Rainbow Dash is a mythic Legendary Creature — Pegasus card from Secret Lair Drop (SLD), collector number 1540. Current market price: $59.97 (foil $73.96), TCGplayer market prices via Scryfall.
Flying, haste Whenever a creature you control with flying and/or haste attacks, you get 20% cooler. (You start at 0% coolness.) Sonic Rainboom — {T}: If you're at least 100% cool, add {W}{U}{B}{R}{G}, draw a card, and reset your coolness.