The fire was reported at 3:37 a.m. at Foxwood Farms, about 15 miles northwest of Chapel Hill.
The blaze had burned through the barn and the horses were dead by the time the first of about 40 volunteer firefighters arrived about 10 minutes later.
"It was totally burned up. There wasn't no roof left," Orange Grove Fire Chief Tommy Holmes said.
Holmes said the horses were likely owned by people who boarded thee animals at the farm, which also has a riding track.
Nine horses from an adjacent stable were saved.
The blaze also consumed about five acres of woods on the property, which ignited easily after days of low humidity and steady winds, Holmes said.
"The leaves or any material in the woods burn real easy right now," Holmes said. "The tops are dry as powder."
The cause of the fire has not been determined. The county's fire marshal is investigating. Chief Holmes said the State Bureau of Investigation brought a dog to the scene to check for the presence of any fire accelerants.