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.
At about 4 p.m., more gunfire was heard in the hills, and the sheriff's helicopter and deputies returned but found nothing.
The woman's husband raised the horse, which she described as a "very youthful 17, a graceful animal, very kind and a good mind."
Animal control officers took the horse, and a necropsy was expected to be conducted.
Sonoma County Animal Care and Control Director Amy Cooper said Monday she could not comment on the open investigation.
Animal Legal Defense Fund spokeswoman Megan Backus Monday said livestock shootings, sometimes involving cows or sheep occur, but are uncommon and typically involve youths.
"The fact this is a high-powered, long-distance rifle shooting, it's just very weird," Backus said.
The nonprofit agency issued a press release Monday afternoon expressing shock at the shooting. The release pointed out California law calls for a person convicted of maiming or killing an animal can be imprisoned up to three years and fined $20,000.
"Residents of Santa Rosa have a legitimate cause for concern when criminals are on the loose who could shoot a kind and gentle horse for no reason," said Stephen Wells, executive director of the animal league.
Backus asked anyone with information to contact the Animal Legal Defense Fund at 795-2533, extension 1010.
Sheriff's Lt. Glenn Lawrence asked anyone with information to contact detectives at 565-2185.