Buck and Monkeyman

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Buck and Monkeyman is an Easter Egg that can be found in the Halo 3: ODST epilogue.

Walkthrough
The egg takes place during the epilogue cinematic, after the final level of Halo 3: ODST, "Coastal Highway". When Vergil lights Avery Johnson's cigar during the interrogation, push the right thumbstick to the left on the controller and the scene will pan over to the left. You will see Edward Buck touch a Caveman similar to the ones in the first level of Halo 3, "Sierra 117". He will then groom it, and appear to eat something taken from its hair; however you will only see him raise his hand towards his mouth, as the scene will black out immediately thereafter.

Trivia

 * Both the Cavemen and Buck and Monkeyman Easter Eggs are mentioned in a Cracked.com article called 7 Creepy Video Game Easter Eggs You'll Wish Were Never Found.