We've all heard of "Hemingway" cats with extra toes. Does it happen in horses?
Yes, if rarely. Alexander the Great's horse, Bucephalus, was supposed to be polydactyl.
In the early 1900s a horse called Norfolk Spider had extra, vestigial hooves on both front feet. He had a common form of equine polydactyly, in which an extra toe grows from the splint bone (which is what's left of a toe) on the inside or outside. Extra hooves also sometimes depend from the fetlock.
These days, extra digits are generally surgically removed, as they cause nothing but problems for the animal. Some, however, are so vestigial they may not be noticed, especially on horses that carry a lot of feather.
If you encounter a mention of a "horned" horse in literature, it means one with extra toes not a horse with a horn (although some horses do have highly vestigial horns). In some cases, this caused confusion and people thinking the "horned" horse was, in fact, a unicorn.
Extra toes are more common on the front feet than the hind.