Fell the Profane // Fell Mire [Modern Horizons 3] – Trendy Electric Mobility Gear for Modern Riders

Aura and Equipment spells you cast cost {X} less to cast, where X is Golden-Tail Trainer’s power.
Whenever Golden-Tail Trainer attacks, other modified creatures you control get +X/+X until end of turn, where X is Golden-Tail Trainer’s power. (Equipment, Auras you control, and counters are modifications.)