It has been suggested that Sleipnir having eight legs is symbolic of the four men who carry a coffin, i.e. a steed to carry the rider into the underworld. It might also be a reference to a real horse with three toes, a genetic manifestation that occasionally happens on the front or rear legs of a horse, usually without harm to the animal. Though rare, it has been seen, and this might have worked its way into the myth.
A stainless steel statue of Sleipnir is a prominent feature in the Midlands town of Wednesbury (which means Odin's fort).
Alternative: Sleipner