Her cutie mark references the alleged doomsday that was coming when this pony's design was being made: 12:21 and the year 2012. Card info:
My Evil Pony Doomsday
DARK/Anti-Pony/4/ATK 1600/DEF 1200
Once per turn, you can Special Summon 1 "My Evil Pony" monster from your hand in face up Defense Position. Monsters Special Summoned this way cannot be destroyed by card effects when they attack. Remove from play the monster that destroys this card by battle and sends it to the Graveyard.
Secret Rare/Ghost Rare