As Norse Mythology tells us, one day, Loki had an affair with Svaðilfari, and so they had a nice baby, the eight-legged horse Sleipnir. (Well, it was actually Loki who gave birth to Sleipnir, and the real myth isn’t as romantic.) Sleipnir happened to be the best of all horses. What’s more, he was Odin’s gorgeous horse.
If you want more information about this affair, and all the ins and outs about eight-legged horse birthing, please read the Prose Edda, and/or fetch a Norse Mythology book 🙂