Do you know for sure it's their dog and it has tags? If it doesn't you could certainly contain the animal and call animal control for a dog on the loose.
No one would question an unknown dog entering and attacking animals on your property. But when it's someone else's known dog, that's a problem. It's treated a whole different way.
Not really any difference at all - in both cases you can call animal control to have the dog picked up as a "dog at large" (being owned and tagged does not change the fact that it is at large or subject to pick-up) - and a dog actively attacking livestock is a dog actively attacking livestock in the eyes of the law- no matter if it is tagged or not.