Well, considering that Sleipnir was an allegory for the funeral procession that people made when carrying the coffins (four people carrying the coffin on their shoulders), I'd say it was likely either a mistranslation or liberal interpretation of Odin's title of god of the dead.
Ummm..yeah... Except that the Norse, Germanic, or Slavic tribes where the imagery originated from
did not use coffins.
That's a christian custom that appeared
well after the image/concept of Sleipnir was established.
The most logical explanation I've come across is that as the best horse available to Man ( Svadilfadi, the father is a Jotunn horse and is not in the realm of Man, and Loki as the mother is...well.. Loki.. A Jotunn itself.) peeps simply did a thing that you encounter in other mythologies as well: double up on the important aspects. Which in case of a horse is its legs. Cerberus got multiple bitey bits. As did the Hydra. Plenty of examples worldwide: Supercritter = more of the things that makes it what it is/makes it useful or dangerous.
Doesn't mean the eight legs haven't been
associated with various, often competing, opinions/allegories over the ages, but the "invention" of the horse goes much further back than we have written sources or depictions, so it's impossible to tell exactly what the original storyteller/priest was on when he thought that one up.
Similar, Odin wasn't God of the Dead. Or but one of them, depending on how you interpret stuff from the Eddas.
Peeps forget ( or simply don't know) that Freya got half of the
warrior souls, and other gods got
their allotment according to their sphere of influence. That rainbow bridge and apple tree didn't guard itself.. Nor did Friggs fields tend themselves..
The common theme was merit/accomplishment in life got you a place to do what you were good at in the afterlife, and not everyone was a battle-crazed musclebrain..
Hel got the Liars, Oathbreakers, Cowards, and Nithings, but pretty much everyone else got some kind of rewarding afterlife, according to their inclinations and profession.