Oh yes if they know they can bully something they sure will it was going on here with my dogs and the geese mainly the gander. One day he had pushed my middle size dog too far and she grabbed him by the neck. Thank goodness I was there to stop any further aggression. From then on my geese and dogs are always supervised and if the geese start towards the dogs with head down they get a pole put between them and the dogs that usually stops them. As long as your gander knows he can scare and upset the horse he will keep it up..Only thing I can see is to keep them locked up or wait till the horse tramples him trying to get away and hope he lives to learn a lesson.