The following is based on Van Buren and has not been confirmed by canon sources.
ARGOS is the robotic juggernaut controlled by ODYSSEUS, the central computer of the prison.
ARGOS is an enormous building-sized multi-legged rover armed with missiles, extremely accurate long-range laser weapons, and an advanced array of sophisticated sensors used for communicating with ODYSSEUS and tracking and finding prisoners via their Lil' Pip 3000s.
In the heist pulled by Victor Presper's men, ARGOS was damaged on its release. This weakened it slightly by damaging its movement servos, and made it impossible for ODYSSEUS to command. ARGOS ceaselessly attempts to retrieve the prisoners no matter what ODYSSEUS tells it to do. This is bad because ODYSSEUS needs ARGOS to seal an enormous breach in the prison through which the PCs can repeatedly walk right out of. Further, Cell Block 13 is completely blasted open, so there's absolutely no barrier for the PCs if they get re-captured. At least the other cell blocks can hold the prisoners that the Prisoner/ARGOS recover.
Argos is designed to hunt down and recapture prisoners that go missing from Tibbets Prison, potentially helping or hindering the player.
ARGOS has not appeared nor is it mentioned in any published game. It was to appear only in Van Buren, the canceled Fallout 3 by Black Isle Studios. It is not certain if it will appear in any future Fallout game.
Behind the scenes
Argos, in Greek mythology, is the loyal dog of Odysseus.