I agree with Joed. When they threatened you then, while they were still there, I'd call the cops.
Have the cops let them know that if anything happens to you or the horses they will be the main suspects.
You could also try to get a restraining order.
Now, all this doesn't help if a horse is hurt. Might be best to move.
For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places.