A special rainbow-colored pony sitting on a prism, which is also her cutie mark. Here's her card info:
My Little Pony Force Rainbow - The Spectrum
LIGHT/Pony/10/ATK 4200/DEF 3600
This card cannot be Normal Summoned or Set. This card cannot be Special Summoned except by the effect of "Force of 7". This card inflicts "Piercing Damage". Once per turn, you can Special Summon 1 "My Little Pony Force" monster from your hand or Deck.
Ultra Rare/Ultimate Rare 
Her revised text negates the effects of the "My Little Pony Force" monster she brings out and elaborates on what "Piercing Damage" is.