Brineborn Cutthroat is a uncommon Creature — Merfolk Pirate card from Foundations Jumpstart (J25), collector number 291. Current market price: $0.20, TCGplayer market prices via Scryfall.
Flash (You may cast this spell any time you could cast an instant.) Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on this creature.