Had the vet out yesterday, just total liquid diarrhea in my 19 yo Arab/QH gelding. Bloodwork fine, fecal didn't show a parasite issue, no fever, respiration fine, no dehydration. He gave him a bio sponge, electrolytes, probiotics and banamine. He did some decent poops yesterday and doesn't seem so bad today but I see diarrhea down his legs again this afternoon :-( Here's hoping it's just some passing virus and nothing to dangerous.
Check around in his paddock or turnout to see if he's licking up dirt - this time of year a LOT of horses, especially if on sacrifice area confinement, will start digging for any roots they can grub along the fenceline, and the resulting ingestion of dirt will definitely cause diarrhea.
We give psyllium (sand clear) product every month during winter months, and giving it in his regular feeding for a week right now cannot hurt.