The Exterminator is an enemy in Chrono Trigger that first appears in Arris Dome in the Future. According to the Charcter Library they were originally made as mouser machines and for this reason they are always after rats. An improved model is the Deverminator.
These enemies appear in one of the full-motion videos of the PlayStation release as well. Lucca, trapped by a dozen or so, is outnumbered and unabled to fend them off. Robo comes to rescue, clobbering them with his Mechanical Arm.
Battle and Strategy
In the subterranean levels of the Arris Dome, where these robots are fought, a thick fog deters vision. During the battle, instead of attacking normally, they respond to the movement (likely through motion detectors somewhere on their anatomy) and project lasers at the object in motion. Because these creatures fight alongside Rats, when a Rat moves, the Exterminator will shoot lasers at it instead of Crono, Marle, or Lucca. In addition to this, they occasionally mistake player movements for Rat movements and will also hit the Rats. Due to the low Hit Points of the Rats, they die from one laser beam. An easy tactic is to postpone attacking until the Exterminator has killed the Rats, then close in on it.