There is evidence that fluoride poisoning kills horses.
After moving to a location where the water was fluoridated, Cathy Justus's horses began experiencing dental fluorosis, acute lameness, hoof abnormalities, bone structure changes, dermatological reactions, colic and behavior changes. Dr. Lennert Krook, Professor Emeritus, Department of Biomedical Sciences at Cornell University in New York, confirmed that her horses were suffering from acute fluoride toxicity or poisoning.
Reverse osmosis filters, activated alumina defluoridation filters and distillation filtration will remove fluoride from water.
Water sources can and should be tested for fluoride. Water Test: Health Guard 1 is an inexpensive test kit which screens for many contaminants including fluoride.