Tail Swipe is a rare Instant card from Store Championships (SCH), collector number 15. Current market price: $0.40, TCGplayer market prices via Scryfall.
Choose target creature you control and target creature you don't control. If you cast this spell during your main phase, the creature you control gets +1/+1 until end of turn. Then those creatures fight each other. (Each deals damage equal to its power to the other.)