Cut Halo 3 vehicles
From Halopedia, the Halo wiki
Several corporate starships were concepted by Isaac Hannaford for Halo 3. He described them as used by a corporation, and said the ships were a mix of military and civilian themes. They are equipped with limited weapons and transport mercenaries and scientists.
Medical Troop Warthog
A Medical Troop Warthog was a variant of the M831 Troop Transport Warthog that was to appear in Halo 3. It was cut from the game during development; however, its files remained in the game's code and can be accessed through modding. It was first discovered in the files of the Halo 3 Beta and is present in the final game as well, although it was never used. Interestingly, Marines will sit on the stretchers like they would do on a normal M831 Troop Transport Warthog, suggesting that it was never fully completed.
Presumably, this vehicle's role would have been to transport wounded troops off of the battlefield to an aid station. It was therefore likely intended to be used during cinematics rather than during gameplay.
Early anti-air Wraith
In an early beta build of Halo 3 dated March 2007, a very early version of the Anti-Air Wraith can be found. Unlike the version found in the final game, this early AA Wraith is covered in a purple sheen like the regular Wraith, and lacks its fuel rod armaments. Instead, the early AA Wraith boats a pair of top-mounted automatic plasma cannons, though they share the same design as what would become the fuel rod cannons. The vehicle can be found on the map Snowbound in this beta build.
Mantis/ walker Shade turret
Referenced only as a physics model within Halo 3's code, the "Mantis" (not to be confused with the Mantis AA gun from the campaign or the Mantis mech from Halo 4) appears to have been a design for a cut mech of some kind. The physics model present within the Halo 3 code references parameters for walking and manipulating legs. It is likely that these entries correspond to the early cut version of the Shade turret found in the March 2007 beta for Halo 3. This early model Shade can be spawned in Forge on the map High Ground, and resembles a walker with six legs, a seat and a plasma cannon. The plasma cannon has a very high rate of fire and can quickly destroy a Mongoose, though the overall movement speed for the Shade is slow, and it cannot climb stairs very well.
Much like the Enforcer in Halo 2, the cut enemy Guardian Sentinel was to work - behind the scenes - as a vehicle with AI. As such, properties for the cut Guardian enemy remain in the game files for Halo 3 as a vehicle - the enemy was planned to be present on the cut level Guardian Forest.