Well if they shot your horse for being on their land, would you be upset? Wouldn't they have a right to remedy your horse/your property being on their's?
As I stated above, it is up to you to fence animals
out of your yard/land, if you shoot someone's livestock you will probably pay about 4 times what they were worth, plus one hefty fine! If it's open range, and many places are, you will go to jail on top of buyin' said livestock! Hell if ya hit one on the highway, you not only will have to pay to get your car fixed, but pay for the horse/cow too!
So what part of ya have to fence animals
out did ya not understand?
To put it in simpler terms, if you and I share a property line with a fence on it, it's as much your responsibility as it is mine.
Hope that clears it up.
(edited to add)Oh, if ya shoot my Angus/Buffalo cross bull, I will
own your property and y'all will never have to worry about it again!