The Guardian was a Forerunner Sentinel located at the Ark.[1] They bear a strong resemblance to the S'pht Compilers from Marathon and the Forerunner hologram in Epitaph.
The Guardian Sentinel was originally set to appear as a fighting enemy in a campaign level that later became a multiplayer level Guardian but was removed, and featured in The Art of Halo 3 instead. It was stated that the player would have been able to rip out the Guardian's eye beam and use it as a weapon in the campaign level.[2]
Although this Sentinel was cut from Halo 3, two Guardians are seen escorting a Keyship to The Artifact in Origins. It is possiable that the gaurdian sentinal was a protecter of the ark