From Halopedia, the Halo wiki
HJ3-213 was a specialized UNSC M808B Scorpion tank that was built for rough terrain, and thus featured thicker treads and a sturdier chassis than the standard M808B. As well as its modifications, the tank had a painting of a black circle divided in half with an arrow pointing down on its left front tread cover.
The tank's serial number and decal can be found in the files of the Halo 2 (Windows Vista) map editor. However, the tank itself never appears in the game. It is possible that it was intended to be one of the three tanks overtaken by the Flood in the level Quarantine Zone, but for some reason it does not appear, and instead two of the three tanks will often bear the same ID.