More than 100 animals have been suffering from severe neglect at a failed riding school about 200km north of Perth, including more than 20 starving horses that have been seized by the RSPCA.
Chief inspector Amanda Swift said a member of the public complained to the animal welfare organisation after observing the poor state of 56 horses on the 2000 acre property.
The owners say they've been there for several years, with some of the horses six years old.
"I would anticipate it was a business venture that went wrong," Ms Swift said.
"They moved out to the country and set up a riding school, but ... these animals require a lot of looking after, a lot of expertise and it's not something you can go ahead and do on a whim."
She said the horses had water but no food and were interbreeding, with malnourished mares being continually impregnated as they weren't separated from the stallions.
"No grass, solid ground, just running free over the acres," Ms Swift said.
The horses have wounds as a result of the males and females not being separated, and may have a range of other health issues.
The RSPCA has removed animals that need immediate care from the site, with stallions taken to a Gingin property, while mares and foals have gone to the organisation's Perth facility.
Other animals in a less dire state are either in foster care or remain at the property.
The owners are co-operating with authorities as they face potential prosecution, with maximum penalties including imprisonment.
Ms Swift said there were also dogs, sheep, chickens and other animals at the property, all of whom are receiving veterinary attention.
She said it had been an emotional time for staff, who had been on site for 10 days and had seen horse carcasses.
"There were skulls and bones, and stuff like that.
"You wouldn't have them laying there unless they died in situ."
It's hoped the ailing animals can fully recover and be re-homed.
Ms Swift urged people in a similar situation to reach out for help and also said those keen to keep horses should understand they are expensive to care for.
"Don't leave it so you find yourself in a similar situation where you've got animals suffering and potentially face prosecution. Make attempts to re-home the animals yourself if you can. Ask for help."