Some years ago when I was in the business of breeding horses I noticed that low-flying helicoptors spooked them far more than low-flying aircraft.
I noticed this on a number of occasions when agricultural aircraft were working paddocks on an adjoining property. When helicoptors were used the horses were considerably upset and on one occasion a mare lost a foal she was carrying after bolting and colliding with a fence post. When aircraft were used the horses hardly lifted their heads despite the aircraft climbing out overhead and then reversing their run.
Perhaps it was due to the rotor noise, a different harmonic, but I could not be sure. Whatever it is a tragedy for the riders concerned to die in such a fashion. My condolences to their relatives and friends.