Unstable Mutation (UMA) from Ultimate Masters, available at 401 Games Canada.

• Rarity: Uncommon
• Finish: Normal
• Artist: Slawomir Maniak
• Card Text:
  Enchant creature
  Enchanted creature gets +3/+3.
  At the beginning of the upkeep of enchanted creature's controller, put a -1/-1 counter on that creature.