Guardian Sentinel

The Guardian was a Forerunner Sentinel located at the Ark. They bear a slight resemblance to Compilers from Marathon, a previous Bungie game. They were set to be seen in Halo 3, but were removed, and featured in The Art of Halo 3. They resemble the hologram in the Halo 3 Multiplayer map Epitaph. They are believed to be the, or similar to the Guardians at the top of the towers in Sandtrap and Sandbox, though no confirmation exists so far.

The Guardian Sentinel was originally supposed to appear as a fightable enemy in a Campaign mission that later became the Multiplayer map Guardian. The player would have been able to rip out the Guardian's eye beam and use it as a weapon.

Trivia

 * The Guardian Sentinel has a striking resemblance to the large hologram in Epitaph