Cavemen

On the Halo 3 level, "Sierra 117", you can find a family of cavemen with Marcus Lehto's face.

Guide
To find the family of Caveman (or monkeys/apes as some people call them), go past the point where Johnson says his Pelican got shot down. You will see a concrete column with the number 3 on it. You stay beside the left wall and go as high as you can, you must turn around and you will see the Cavemen against the wall. The largest of them is holding a teddy bear. Although you can try to shoot them, a death animation for them has not been made, so they will remain moitionless if you try to kill them, though blood will still spurt out.

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The Lone Monkey Man
Recently found, there is a monkey man sitting on a cliff outside the level, in the area where you rescue Johnson from the covenant. He is accessible by means of a glitch, but he is still visible by using a scoped weapon such as a sniper or the binoculars.