Brumaks are very large apes that were bred by the Locust. The Brumak naturally grow to be 15 meters tall and weigh up to 15,000 kilograms. They have twin Troika turrets attached to each of their wrists and a large missile launcher on their back. They are controlled, like most creatures from the Hollow, by some sort of computer technology implanted into their nervous system, allowing a Locust driver to have full control of the Brumak. Across Sera, Stranded and COG have been reporting Brumaks that are outfitted with energy weapons rather than the iconic missile launcher.
The Brumak made its first appearance in Gears of War during a cutscene in the fourth act, where Marcus Fenix and Delta Squad flee from the beast. Many players were disappointed that the chapter where the player actually engages this colossal foe in combat had been cut from the game, so when Epic released the PC version of Gears of War they went back and completed development on the unfinished chapters leading up to and including a fierce boss fight with the Brumak.
The Brumak plays a much larger role in Gears of War 2. Marcus fights several of them throughout the game with the help of vehicles, mortars, and Troika machine guns. Eventually, Marcus and Dom take control of a Brumak (trapped in the burning remnants of a building) themselves to wreck havoc on Locust forces. After coming in contact with Imulsion, the Brumak eventually turns Lambent. It is dealt with quickly, by Hammer of Dawn strikes, and provides another huge explosion that helps sink Jacinto and flood the Locust tunnels.
In Gears of War 3 the Brumak appears as a boss in Horde mode, and also appear several times in the main story. When the Hollows were flooded, it is thought that the Brumaks were made extinct. But a few stragglers remained for the Locust to again enslave, breed, and turn into walking tanks. The first Brumaks appear after Sovereign is sunk by the Lambent Leviathan. One is killed by a Siegebeast, which had been hijacked by the player, and the second is dealt with on foot. Another Brumak is faced when Delta Squad drive through an airfield with Dizzy. The Brumak is quickly dealt with, rammed by the truck and decapitated by some razor wire.