A few weeks ago this corner of the corral was a pond. The addition of some fill material made things better but it was still wet, suggesting there was another source of water.
You might point to the horses in the shelter, and you’d be right, but the hose used to fill their water buckets, seen in the photo, was leaking. That was fixed on December 9.
How do you know if you addressed the cause of a problem? It goes away.
Getting rid of the horses is not an option. Throwing out some kitty litter to absorb the moisture doesn’t fix the leak and only prolongs the problem.