I think it’s fair to say that horse’s have a wider range of personalities or temperaments than motorbikes do. A bike won’t throw you off if you’re doing everything properly. A horse on the other hand can just not be arsed carrying you anymore or get spooked by something passing by.

All that said I don’t think it would be easy to build a credible frame of comparison between the the two forms of transport. I wouldn’t be surprised if either could be ‘proven’ to be more dangerous given the amount of thin air that the frame of reference would be resting on.