Crook County Sheriff's Sgt. Travis Jurgens said deputies who went to the area Friday with law enforcement officers from the Forest Service and Bureau of Land Management to continue the earlier investigation came upon the additional carcasses.
No details were proved on the latest horses.
The animals were killed in the Ochoco National Forest about 30 miles east of Prineville. Hunters reported finding the first three horses.
Forest Service officers went to the area last Sunday and found two adult horses and an adolescent.
KTVZ-TV reported that Forest Service Capt. Dan Smith said one of those initial horses was found barely alive with mortal injuries and an officer euthanized it.
In the spring of 2011, six wild horses were fatally shot in the Ochocos. No arrests have been made in that case despite a $4,000 reward.