Brineborn Cutthroat is a uncommon Creature — Merfolk Pirate card from Foundations (FDN), collector number 152. Current market price: $0.22 (foil $0.32), 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.