Someone armed with a high-caliber rifle shot a horse as it stood in a pasture in the hills above Bennett Valley on Sunday morning.
The wounded animal sprinted back to its stable and died.
"It's just devastating," said the fearful owner, who spoke on condition her name not be used.
A $5,000 reward is being offered by the Animal Legal Defense Fund for information leading to the arrest of the shooter, agency officials reported Monday.
A Sonoma County spokeswoman for the national nonprofit animal advocacy group called the shooting disturbing and rare.
The horse's owner said Delta Fox, a 17-year-old male American warmblood, and two other horses had been out in a pasture together at about 8:30 a.m.
"They came racing up the hill in a huge panic," the owner said. "One of them entered the stall and collapsed. My husband went over to see and found a hole through the horse's flank."
The couple called Sonoma County sheriff's deputies for help at about 9:10 a.m.
Sheriff's deputies, county animal control officers and the sheriff's helicopter responded. They conducted an extensive ground and air search of the rugged terrain for a shooter or signs of a shooting, sheriff's officials said.
The property is close to Annadel State Park. It wasn't known whether the shooter fired from park land, but it appeared the horse had been shot from a distance.