Since it has a collar I would try to find the owner once and if it comes back, well, sometimes dogs go missing. If it looks or acts mean, I would skip the first warning. If you have kids in the area, I'd get rid of the dog.
When I was a kid I was picking tomatoes in the edge of my parent's yard and I looked up and 4 dogs were running from the woods towards me. I started yelling and running back to the house and my dad came out of the garage with a baseball bat. I'm not the fastest runner, but I covered some ground quickly that day. I don't think I've seen my dad move that fast either. One big dog made a loop towards my dad and he swung for the fences and hit a hind leg that shattered. The other dogs took off and the big dog took off on 3 legs before my dad could finish him off. I'm not sure what the dogs were doing and maybe my dad could have just yelled at them to make them leave, but I'm pretty sure that day he was trying to kill them all. We never saw those dogs in the yard again, but 25 years later the red Louisville slugger still sits in the same corner of the garage.