A Banana Coin is an item that appears in throughout the Donkey Kong series.
Donkey Kong Country 2: Diddy's Kong Quest
Donkey Kong 64
Banana Coins are very common in Donkey Kong 64 and come in five different colors: yellow for Donkey Kong, red for Diddy Kong, blue for Lanky Kong, purple for Tiny Kong, and green for Chunky Kong. Collecting these items are required as they allow the Kongs to buy potions from Cranky Kong, instruments from Candy Kong, and weapons and gear from Funky Kong. Finding a rare 5-Banana Coin adds 5 regular Banana Coins to the Kong that picked it up.