Unstable Mutation is a special Enchantment — Aura card from Time Spiral Timeshifted (TSB), collector number 33. Current market price: $0.19 (foil $2.42), TCGplayer market prices via Scryfall.
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.