Retriever Sentinel

The Strato-Sentinel was a Forerunner Sentinel variant. They were designed to strip mine the resource moon located in the middle of Installation 00, to help in the the construction of a Halo ring, such as the newly constructed Installation 04 as seen in Halo 3's The Covenant and Halo, and were also used with the construction of the Portal on Earth, in the present-day country of Kenya.

One of the concepts for the multiplayer level Blackout had the map set in the partially destroyed insides of a Strato-Sentinel orbiting the Ark's strip mining moon.

Trivia

 * Strato-Sentinels were originally planned to appear in Halo 3, but wound up being cut from the game.
 * The word "Strato" in the Sentinel's name is most likely derived from Latin, as "Strato" is the ablative form of "Stratus", meaning "stretched out" or "extended".