Spitting Slug is a special Creature — Slug card from Time Spiral Timeshifted (TSB), collector number 85. Current market price: $0.31 (foil $1.08), TCGplayer market prices via Scryfall.
Whenever this creature blocks or becomes blocked, you may pay {1}{G}. If you do, this creature gains first strike until end of turn. Otherwise, each creature blocking or blocked by this creature gains first strike until end of turn.