Hidden gargoyle

The hidden gargoyle is a Halo 2 Easter egg that can be found in the multiplayer map, Backwash.

Walkthrough
To find the hidden gargoyle, head over to the Blue Base and look towards the Forerunner structure. The figure is located on the left side of the roof, slightly above the balcony of the structure.

Note : This object would be more accurately described as a "Grotesque", as a Gargoyle is used to redirect rainwater while a Grotesque is simply decorative.