Tail Swipe is a uncommon Instant card from Foundations Jumpstart (J25), collector number 721. Current market price: $0.29, 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.)