Monkey on Sierra 117.
From Halopedia, the Halo wiki
In the first level of Halo 3 after the pelican is shot down procced through the level untill you reach the spot with the Jackles yeilding carbines.Hug the left and climb to the top of the position of the Jackle snipers, in the corner there should be a four to five monkeys of sizes that vary, the biggest is holding a teddy bear.
