Well I sure understand why you are upset. If you can't find the owner the best thing to do would be to turn him over to the shelter. Thats what they are for. A intact male wandering the streets can get into big trouble. Dog fights, hit by a car, who knows... If the owner cared so much , they wouldn't let their dog run loose, period! If you turn him over, they should know where to look to claim him. If you do find the owner, I think you could recover at least medical expenses. My friend had the neighbor's stud got loose and came to her house and breed her mare. She bred Apps for a living and it was a business, and filed her taxes as such. She got loss of income and medical paid by the neighbor. Pet/animal owners are responsible for their animals and what they do not matter if it's a mistake or not. You made a mistake by leaving her alone, but on the other hand if there is a leash law in your area, you shouldn't have to worry about other dogs coming on to your property. Good luck catching him and hope your girl is ok.