Bob Henderson, left, examined one of the shot wild horses on Wednesday. (CBC)

A reward for information leading to the arrest of the killer of wild horses in central Alberta has doubled this week to $21,000.

The Wild Horses of Alberta Society had a reward fund of $10,000 that was collected in 2007 after the carcasses of two foals and a mare were found west of Sundre.

On Tuesday night, a pregnant mare, a stallion, and a yearling colt were found shot to death near the Mountain Aire Lodge motel and campground west of the town. The RCMP believe their killer fired from the road 500 metres from the mountainside where the animals were found.

Evidence at the scene indicates that the mare went into labour after she was shot, and that the stallion, who was found on his back tangled in some trees, suffered an agonizing death. The deaths bring the total number of wild horses killed in Alberta to more than 22 over the last four years.

The owner of the Innisfail Truck Ranch pledged $5,000 to the reward fund this week, telling CBC News it's for a "good cause."

"This has all happened once before out there. I just feel like there's some maniac running around out there just shooting at anything. I think it's important to just catch him or her," said Daryl Czuy on Friday. "It's senseless shooting is what it is."

The Alberta Equestrian Foundation has also pledged $5,000, while the SPCA has committed $1,000, bringing the total reward fund to $21,000.

Feral horses not considered true wildlife

"It is heartwarming to see groups and companies respond with their support. It assures me that we are doing the right thing and that I should never give up in our effort to save the wild horses," said Bob Henderson, president of the society.

The Alberta government estimates there are about 300 feral horses in the Sundre area. Provincial biologists don't consider them true wildlife because they originated from domestic horses used in logging and mining operations in the early 1900s.

The society wants the government to recognize the feral horses as wildlife, so that they have more protection. Anyone convicted of killing cattle, which is the category feral horses fall under, faces a maximum sentence of five years in prison under Canada's Criminal Code.

With files from Andree Lau