Please! The horse by definition is mammal, and mammals are, by definition, four-limbed vertebrates. How, for Loki's sake, the eight-legged creature could be relative to the horses?
"For Loki's sake", just as you said?
The genetic basics of morphology are a quite active field or research right now. To simplify, a few
key genes create gradients of certain chemicals across the growing body, which then serve as a coordinate system for the cells, triggering genetic "programs" quite close to "oh, I'm supposed to form a leg here". Mess with those gradients and your definition of four-limbed-ness goes
bye-bye. And the womb Sleipnir formed in supposedly was imbued with divine capabilities ...