Has your horse had a change of diet or routine recently? This kind of problem is most common when horses encounter the young spring grass and also spend more time grazing. Water has up to 4 times the moisture content of hay, and so its not unusual to find a lot more water in the waste. Its important at any point that there is a change in diet to introduce it gradually. However, even if you don’t most horses should recover from this condition in several weeks. You can also consider a course of probiotics if the condition persists.