(plaintiff sustained injuries during horseback ride on guided trail ride at defendant's ranch when horse plaintiff was riding brushed up against tree injuring plaintiff's leg and hip; trial court denied defendant's motion for summary judgment on basis that defendant's evidence raised question of fact concerning whether they knew of horse's propensity to walk too closely to trees; appellate court affirmed trial court's denial of summary judgment to defendant; riding lesson also ancillary to recreational activity of horseback riding and, as such, release of liability void as against public policy; defendant failed to establish that plaintiff assumed risk of horseback riding as a matter of law because question of fact raised as to whether defendant unreasonably increased risks of horseback riding by using bitless bridle on horse; plaintiff novice rider).