The Humane Society of the United States says it is offering up to $US2500 for information leading to the arrest and conviction of those responsible for the attack in McConnelsville early last Sunday.
The reward adds to the $US5000 already offered by the Blue Ribbon Arson Committee.
The Morgan County Sheriff's Department and State Fire Marshal said property owner Brent Whitehouse woke early on Sunday to check on Love, a pregnant quarter horse due to give birth, only to find the barn housing Love and seven other horses engulfed in flames.
All of Whitehouse's horses, including Love and a week-old foal named Buddy, were killed in the blaze. The fire marshal ruled the fire as arson.
Karen Minton, Ohio state director for the humane society, said: "We are outraged by the intense suffering of the eight innocent horses that burned to death and we hope our reward helps investigators find the person or persons responsible for this callous act."
Anyone with information on the crime is asked to call the state fire marshal at (800) 589-2728, or the Morgan County Sheriff's Office at (740) 962-4044.