Dogs are well known for their ability to detect Infiltrator units. They frequently start barking when an Infiltrator gets close to them.
Ever since Skynet developed T-800 units that used living tissue over a metallic endoskeleton, the Resistance has had a hard time identifying Infiltrator units. As a result, a significant number of Infiltrator units have been able to breach Resistance defenses and slaughter Resistance soldiers. However, the Resistance has found that trained "sniffer" dogs posted at checkpoints are effective at detecting this new threat.
Exactly how dogs are able to identify terminators is not answered in the film. However, since dogs are often seen barking or growling at an enemy in many films, the use of them is a clever plot device here.
- Dogs, which appear to be German Shepards, at the Resistance bunker checkpoint react to a Terminator.
- A dog, another German Shepard, at the Tiki Motel reacts to the T-800.
- Sarah Connor brings an Alsatian dog with her, presumably because of dogs' ability to detect the Terminators.
- Mary Randall recollects patrolling the Future War with dogs and detecting a Terminator in a poncho.
- The puppy Dieter von Rossbach wanted to give to John reacts to the T-800 they destroyed at Connor's safe house in Paraguay.
- In Terminator 2: Judgment Day, the dogs at Enrique Salceda's ranch do not bark when the "good" Terminator is nearby. It is unclear whether or not this is for a specific reason, such as "dogs can sense anyone who comes with bad intention (or mind)", or if James Cameron simply didn't want to confuse audience members by unnecessarily introducing tension into the scene.