Rtas 'Vadum in crashed Flood ship
From Halopedia, the Halo wiki
The Half-Jaw in crashed Flood ship easter egg can be found on the level Floodgate. When you jump down the hole to get into the Flood ship that crashed, there will be a few dead Ultra Elites lying around. The first one you see sometimes has Rtas Vadum's (Half-Jaw) face. However it is the old model from Halo 2. You can get it infected too if you push it down the hole and release some infection forms. But when he turns into a combat form, his armor becomes the default blue of most flood.
http://img119.imageshack.us/img119/2048/13829028mediumhw4.jpg
