Hydra Trainer is a uncommon Creature — Human Warrior card from Modern Horizons 3 (MH3), collector number 159. Current market price: $0.28 (foil $0.34), TCGplayer market prices via Scryfall.
You may exert this creature as it attacks. When you do, target creature gets +X/+X until end of turn, where X is the number of counters on permanents you control. (An exerted creature won't untap during your next untap step.) {2}{G}: Adapt 2. (If this creature has no +1/+1 counters on it, put two +1/+1 counters on it.)