From the Super Mario Wiki
The Rareware Coin is a large coin with the Rareware logo on it found in Donkey Kong 64. It appears by earning 5000 points in Rare's classic game, Jetpac, and collecting the coin in-game. To access the game, players must go to Cranky's Lab and have at least 15 Banana Medals. It and the Nintendo Coin are required to open the door to get the last key for K. Lumsy's cage.