Horse welfare advocate Richard Couto, of the Animal Recovery Mission, said four of the horses - three thoroughbreds and a quarter horse - were found on Wednesday in a field that appeared to be a dumping area.
Couto called the police and the South Florida SPCA.
Couto suspected it was a site for holding horses before slaughter. The horses were thin and suffering from rain scald and sores.
The SPCA found a fifth horse, also a thoroughbred, on the same day in another part of the county. It also appeared to have been abandoned.
The thoroughbreds were placed in the care of Florida Trac, a retirement and placement program for horses, and the quarter horse was with the SPCA.
Cuoto said efforts were under way to trace the thoroughbreds through their lip tattoos to find out how they ended up in their predicament.
Cuoto told local media: "Bad people get a hold of these horses, and they have relationships with the illegal slaughter farms in Miami-Dade," Couto said. "They give these horses to these slaughter houses as a favor or for a small payment, maybe a couple hundred bucks."