Golden-Tail Trainer is a uncommon Creature — Fox Samurai card from Modern Horizons 3 (MH3), collector number 187. Current market price: $0.24 (foil $0.36), TCGplayer market prices via Scryfall.
Aura and Equipment spells you cast cost {X} less to cast, where X is this creature's power. Whenever this creature attacks, other modified creatures you control get +X/+X until end of turn, where X is this creature's power. (Equipment, Auras you control, and counters are modifications.)