What was a story told long ago about a person that visited family in some deep hillbilly lands or swamps and saw a horselike creature creeping through the place during a rainy night kind of thing?

I hate to break the illusion, but… Horses sometimes just graze at night, especially full moon nights. A freshwater swamp offers a wide variety of tasty plants, and freshwater, so it could literally just have been some guy that saw a horse at night.