Well, it depends on what the intended purpose of the unit is.
Drone (obvious, but not copyrighted. You could also say: "Bee" or "Wasp" because those societys contain drones)
Sentinel (another obvious one)
Mechanical Cavalry (I just like the sound of this one)
Unmanned Mechanized Combat Suit (UMCS for short)
Umanned Armored Unit
You could also go the "blunt" aproach:
Unmanned Combat Vehicle
I hope this helps.
Grim Reaper: You are all dead. I am Death.
Host: Well, that's cast rather a gloom over the evening, hasn't it?