A five-year-old thoroughbred named Kinkisi was found with its throat slit and its front legs removed at a ranch in Miami Gardens last Wednesday, becoming at least the 18th victim since the killings began in January.
The animal was found dead in its stall at 7am as an owner was going to feed his animals.
"Words cannot describe the horror of the scene," the SPCA said.
"But the pictures we have are too graphic to post, so I'll try.
"Kinkisi, a well-loved and cared for five-year-old thoroughbred who last raced at Calder, was found with his front legs severed and removed at the shoulders, his throat slit, his chest cut open with his intestines piled on top of his body.
"Deep puncture wounds in his rump disabled him further before the kill.
"If you find that description disturbing, so did the young schoolgirl who showed up at the ranch with her mother to feed her own horse that morning.
"No child should ever have to see such horror.
"In fact, no-one should see this horror committed against a beautiful, helpless, gentle pet.
"No-one should see the heartbreak in the owner's eyes as he knelt next to his horse to say good-bye one last time.
"Please help us stop the slaughter before the next pet is killed. Pass the word."
The SPCA said horsemeat is illegal to buy or sell. It is not safe for humans to eat, nor is it ethical to eat someone's beloved pet, the branch said.
The reward for information leading to the capture of those responsible now stands at more than $US22,000.
A rally was held at the weekend calling for an end to the slaughter.