Medical Troop Warthog
From Halopedia, the Halo wiki
The 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.