Sorry to kibitz so long after the fact, but Sleipnir like many critters in various mythologies was symbolic. In this case he represented the guys who carried a dead person off to be buried, which in Norse was four guys, one at each corner of the bier. Four guys have eight legs. Odin amongst other things was god of the dead heroes - his riding around on a horse representing the four guys that carried a dead person to burial was symbolic of that.
I'm pretty sure it was eight-legged. I've always thought it had eight legs, and I just glanced around at a bunch of art and websites about it which say it's eight. I personally think he would look better with 6 legs, or better yet, four, but he wouldn't be particularly remarkable then, would he?