Best of Horses

When Norse poetry lists the best of things, the horse on the list is Sleipnir: the Eddic poem Grímnismál puts him at the head of the horses the way it puts Odin at the head of the Æsir. Snorri adds the detail everyone remembers: he is Odin's, he is grey, he has eight legs. That is the whole description. No source explains the legs, defends them, or treats them as needing defence — they are simply the fact about him, stated in passing, the way you would mention a colour. Whatever they meant, the audience already knew.