Via the National Institute of Health, a 1991 study from the British Journal of Sports Medicine identified horse-riding is more dangerous than motorcycle riding.
Horse-riding carries a high participant morbidity and mortality. Whereas a motor-cyclist can expect a serious incident at the rate of 1 per 7000h, the horse-rider can expect a serious accident once in every 350h, i.e. 20 times as dangerous as motor-cycling.