Cinnamontoast
Fais pas chier!
The definition of criminal damage is that
'A person who without lawful excuse destroys or damages any property belonging to another intending to destroy or damage any such property or being reckless as to whether any such property would be destroyed or damaged shall be guilty of an offence.'
I'm not suggesting that you'd get a conviction, but if the police wanted to have words this guy he is damaging the OPs horse, which is legally just property. I'm pretty confidant agrivated tresspass doesnt need to be criminal damage and that causing the OP distress would count.
OH reckons you wouldn't get a conviction but worth a shot to frighten him. Aggravated!! (Missing my job, clearly!)