Sheriff Dave Shipman said 96 of the 215 horses at the ranch were dead, and the department received a search warrant to hold the rest of the animals.
The Bismark (N.D.) Tribune reported only the animals in the worst condition would be removed from the property, while the others would be cared for on the ranch.
Shipman said the department began receiving calls about loose horses and neglect in the past month, and authorities found the dead horses during a welfare check Monday.
The case will be sent to the Morton County State Attorney's Office to review possible charges, police said.
Shipman described the owner of the horses, who wasn't identified, as a retired man with a "soft heart" for horses.
"My personal opinion is he just got in way, way over his head," the sheriff told the Tribune. "He is cooperating with us, and he realizes that there is a problem."
State veterinarian Susan Keller said the incident is rare and the recent cold weather may have played a part in the animals' death.
"You can have abuse and neglect, and death doesn't always result," she said. "Sometimes, timing of lack of adequate feed and water and weather come together. That's probably what happened in this situation."