This aircraft is also on the On-call USFS list. It may well be a survey flight.
US 91-119 says...
An aircraft must maintain an altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.
Wild horse habitat sounds 'sparsely populated" to me....
So...which was more frightened, you or the horses? : :