Sentry bots, or sentries, are walking mechanical monstrosities armed with miniguns and rockets. They are apparently between 8 and 10 feet tall and match the appearance of super mutants.
The sentry bot is a pre-War model, as they can be found in locations that had been isolated from the Wasteland, for example inside the bunker in the lower levels of the Toxic Caves in Fallout 2. Many can be found in the Sierra Army Depot. Compared to the sentry bot found in the Toxic Caves, they are much deadlier adversaries. Capable of packing as much firepower as a super mutant with a rocket launcher and minigun and extremely well armored. Fighting them in the Sierra Army Depot is difficult, but can be done if you take the initiative and shoot out their cameras (eyes). Once blind, they will run around blindly and can be easily destroyed at your leisure. There is another weakness, you can deactivate them if you have a high repair skill, causing them to shut down and yield an easy source of experience. The Enclave is also known to use them, as many of the bots can be found in the Enclave Oil Rig.