The second mutation of the creature known as G, formally William Birkin.
In this second form you briefly see a mutation in progress. All the damage you caused it in the first battle finally catches up with it, and the G-Virus inside its body has no choice but to mutate. In this case its arm grows even bigger as new claws are formed that rip through its flesh. Its once-human head dissolves into its chest as a completely new head grows from between its shoulders. The signature eye is still there on the shoulder of the enormous arm.