There is a myriad of conditions that could be doing this! You MUST contact your vet! It could be very serious. Equine Protozaol Myelitis comes to mind, EHV neurological form, rabies, Vitamin E & Selenium deficiency, Magnesium deficiency, etc! You see how it is not wise to take this lightly. There is danger to your horse and others as well. (People too)Best of luck, I hope it isn’t serious!