First, there are different types of guard dogs, both in terms of breed and in terms of training. This comes down to the type of work the dog is doing, and is basically the difference between alarm dogs and attack dogs.
In fact, many small breeds of dogs were originally bred to serve the purpose of alarm dogs, alerting their owners to things out of the ordinary. These dogs tend to have a rather low bark threshold but lack the physical size and strength to inflict significant damage on an intruder, human or otherwise.
Dogs that are trained as guard dogs are generally used for various purposes:
- To discourage intruders from entering the premises in the first place
- To alert their human handlers to the presence of an intruder
- To run an intruder off the premises, so that they cannot do any more harm
- To keep an intruder in place, so that they cannot do any more harm and can be captured by human guards
and in extreme cases
- To severely incapacitate or even kill an intruder to ensure that they cannot do any more harm
For anything other than merely discouraging intrusions to be effective, the dog must have both the physical strength, dexterity and mentality to face down a threat. A dog that turns tail and runs at the sight of something unfamiliar would not be a very effective guard dog!
Even if the goal is simply to discourage intrusions, it is likely that a dog that turns tail and runs at the sight of something unfamiliar would not be very effective in its job.
As a consequence, guard dog training focuses a lot on giving the dogs the mental tools required to face down a threat and to handle it however the human handlers feel this dog should handle a threat.
This includes teaching the dog to not accept "bribes" from strangers, precisely because if the dog is distracted by such a bribe it does not make an effective guard dog.
That said, young dogs sometimes have what is actually called the "ghost period", during which, to it, everything is potentially scary. This tends to happen around adolescence, but breeds and individuals vary, and tends to pass relatively quickly with simple confidence training.
Thus, a trained, adult guard dog should not turn tail and run at the sight of something unfamiliar. It may have been trained to go get its handler if something unusual happens, but it's more likely that the guard dog would either be trained to work alone, or have its handler along with it, either of which means that won't be a necessary response. Once it realizes that the skeleton is an intruder, the dog should be handling the skeleton in much the same way it would handle any other intruder.
