Phoenix Logs/UNSC Units/Colossus
From Halopedia, the Halo wiki
The Colossus is a superheavy vehicle unit which can be deployed by Sergeant Johnson in place of the Scorpion. It is relatively slow, but fitted with a devastating arsenal, capable of taking down almost any foe. The Colossus' primary weapon is a long-range heavy railgun, a massive weapon originally designed for installation on fortified structures. While the rail cannon can only target ground units, its autocannon has an all—environment engagement profile. After upgrades the Colossus can use Stomp, sending out a shockwave that stuns and damages enemy ground units.
The Colossus is a next—generation exoskeleton combat unit designed by Materials Group and Hannibal Weapon Systems for the UNSC Army. Isabel retains its manufacturing templates from her supply-chain training on Earth, though manufacturing these massive combat machines represents a considerable burden on the Spirit of Fire's supplies. Sergeant Johnson claims to have acquired the materials for his Colossus units via the classic Marine Corps logistical expediency of 'borrowing' equipment that isn't nailed down [or couldn't be pried free], but his actual source for these machines is a mystery wrapped in an enigma, protected by shield generators and several tons of Titanium—A armor plating.