From the Super Mario Wiki
The Rareware Coin is a silver coin with the Rareware logo on it in Donkey Kong 64. To obtain this item, the player must play Jetpac from Cranky's Lab and earn at least 5,000 points. This causes the coin to appear within the game. Both the Rareware Coin and the Nintendo Coin are required for opening a door in Hideout Helm with the final Boss Key for K. Lumsy's cage.