If there was damage sustained to the horse(s) such as running through a fence etc, then the a/c operator would be liable, including a fixed wing aircraft. This would fall under noise polution and aside from hot-air balloons there is no noise polution insurance available. Therefore, it's very much pilot beware.
I know of this after a prized deer ran through a fence and escaped when I legally overflew a property at 500ft agl. Bye bye spare cash.
If the horses are unhurt there is little that can be done other than a polite warning to the land owner perhaps.