The horse in a pasture on Willow Creek Road was shot sometime between 12:45 and 1:15 p.m. Saturday, spokesman Patrick Orr said Monday.
As over the weekend, anyone with information about the shooting is urged to call Ada County police dispatch at 377-6790.
The owners of the 20-year-old horse, Rocky, saw him lying in the pasture on Willow Creek Road. They thought he might have colic, but when they checked on him they saw he had been shot and was near death. He was shot once in the shoulder, police said.
A veterinarian who responded to the scene said the horse had lost too much blood and had gone into shock. He could not be saved.
Investigators said the location of the pasture, the position of the horse and the type of wound suffered led them to believe the shooting was an intentional at.
The pasture is located in a rural residential area that makes it unlikely that a hunter or recreational shooter would have hit the horse accidentally, police said.
Rocky's owners say he was a faithful hunting trail horse and that they are distraught over the loss.