A woman whose face is so hideous that it reminds one of a horse and whose legs will spread open for virtually anyone. Horse sluts upset the balance of the space-time continuum. They should not be able to get laid, yet they do. They also generally make a lot of babies that end up in foster care. A horse slut should be avoided at all costs. You can generally find them in trailers or bars. Usually no one pays them attention until they've struck out with every other possible slut in the vicinity.
Person 1: "See that nasty worn-out looking woman over there? I totally tapped it."
Person 2: "Bro, you should never admit to tagging a horse slut. I hope you used a condom."
Person 1: "Of course I....wait...FFFFFFUUUUUUUUUUUUU..."
A slut that has been ridden by lots of men, much like a horse that has been ridden by lots of people. It's not a term used to describe looks (see horse faced slut). All horse face sluts are horse sluts, but not all horse sluts are horse faced sluts. It's never a good idea to brag about sleeping with a horse slut since most everyone already has and since it means you more than likely caught some crotch funk.
Person 1: Dude, you didn't sleep with Amanda, did you?
Person 2: Of course I did!
Person 1: Dumbass...she's a horse slut. Enjoy the herpes.
Person 2: What? FFFFFUUUUUUUUUUUUUUUU.