Hell Mongrel is a common Creature — Nightmare Dog card from Modern Horizons 2 (MH2), collector number 88. Current market price: $0.22 (foil $0.32), TCGplayer market prices via Scryfall.
Discard a card: This creature gets +1/+1 until end of turn. Madness {2}{B} (If you discard this card, discard it into exile. When you do, cast it for its madness cost or put it into your graveyard.)