RE: Letter of warning to dog / pet owners....
You don't like hunting do you?
In my opinion, unless you are the owner of that land, sending a letter like that would be going outside the scope of permission that you have.
If one of the people that I allow to hunt on my land did what you propose, that person would never have permission to hunt again.
If you actually shot a horse, you would probably have to leave the state..that is if you had any money left after the civil suit and after your house was burned to the ground. You might suffer the same fate with the dog too.