Unidentified human transport ship
From Halopedia, the Halo wiki
This transport ship can be found in the skybox of the Halo 4 Castle Map Pack map Outcast. When first announced the Halo Waypoint screenshot gallery for the map labelled the ship "Darter", initially leading to the assumption that this ship was in some way related to the D82-EST Darter prominently featured in the Halo Wars games. A later comment on this website from Halo 4: The Essential Visual Guide contributor Stephen Loftus clarified that the ship was not a Darter - noting the extreme size difference between the two craft.
List of appearances