Golden-Tail Trainer

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.)

From Magic: The Gathering.

Open the interactive version on https://mythscribe.cloud/app/tcg/cards/cms6o6mup3uzvpb1hajkgk6m4.