I'm just curious, why do you not want horses on the photo? Is it because there's been so many usage with horses? I'm just saying, in my opinion, someone is using your stock and crediting it so that other people can find you and see your work - and you're going to exclude a vast majority of people from doing this. To me, it's a little strange that someone would go "I'm sick of all these horses! No more" like, it's not a big deal. No one cares if it's a bunch of horses.
Unless there's another reason, please let me know.
It's a late reply so I apologize, and probably the reason why some people restrict Horses or in most what I consider "Horse Manipulations" is because the image of the horse can easily be pasted onto any image without the effort of adjustments to make it seem like they blend in / belong into the background. It's not just FrantisekSpurny himself that restricts horses but other people too because of the explanation stated.
Otherwise, I could be wrong since it is an opinion of mines and Frantisek might have a different reason.