In Donkey Kong 64, Diddy, Tiny, Lanky and Chunky are all unlocked by freeing them from cages. While DK appears to be unlocked by default, this is false: internally, the game only unlocks him by opening the door to exit DK Island. The entirety of DK Island is his personal cage.
ALT Source: tcrf.net user "Alley", tcrf.net/File:DK64_YellowQ_TagBarrel.png