The 4-year-old thoroughbred named Kinkisi was found with its throat slit and its back legs removed at a ranch in Miami Gardens.
The animal was found dead in its stall at 7am on Wednesday as an owner was going to feed his animals. The barn houses about 20 horses.
News reports suggest a security guard had been at the barn until 5am.
Miami Gardens police are investigating the latest attack, which has outraged the horse-owning community in Miami-Dade and surrounding areas. The killings have centred on Miami-Dade County, where 17 of the animals have been butchered.
The last known attack was in July.
Authorities believe meat from the horses is being sold on the black market, where it can fetch $US20 or more a pound.
The reward leading to the arrest of those responsible has topped $US20,000, including $US5000 from the Humane Society of the United States, $US3000 from Miami Crime Stoppers and $US1000 from Broward Crime Stoppers. Chris Chiari, a Davie resident running for a seat in the Florida House, has also contributed $2000.
Developer Ron Bergeron has added $US10,000 of his own and Dr Robert Oller has reportedly added another $2000.