Gryffwing Cavalry (VOW) from Innistrad: Crimson Vow, available at 401 Games Canada.

• Rarity: Uncommon
• Finish: Normal
• Artist: Steve Prescott
• Card Text:
  Flying
  Training (Whenever this creature attacks with another creature with greater power, put a +1/+1 counter on this creature.)
  Whenever Gryffwing Cavalry attacks, you may pay 1W. If you do, target attacking creature without flying gains flying until end of turn.