Troop deployment pod

Were you looking for the SOEIV, a human drop pod? The troop deployment pod is an extension which can be attached to a D77-TC Pelican's overhang to increase its passenger load capacity. This extension can seemingly double the size of the craft's "blood tray".

While not seen or directly mentioned in the series, the fact that Pelicans sometimes carry unusually large complements of people in the novels may suggest that the UNSC use some form of troop bay extension. At the end of the the Battle of Sigma Octanus IV, the SPARTAN-IIs were able to evacuate a group of surviving civilians as well as Corporal Harland's squad and the remaining Marines from Alpha Headquarters. Including the SPARTANs, this would make a total of forty-six people, which a normal Pelican cannot ferry. It was also once assumed that a Pelican with a troop bay extension was used to ferry the seventy-five SPARTAN-II candidates; however, the 2010 reprint of Halo: The Fall of Reach and Halo: Fall of Reach - Boot Camp retconned this Pelican into being an Albatross.

Trivia
Early concept art for the Pelican in Halo: Combat Evolved included one iteration by Marcus Lehto in which a carriage was inserted onto the back of the vehicle to make the "blood tray" larger.