Horses in medieval culture often symbolize sexuality, and Sleipnir's no exception. In fact, in "The Walling of Asgard,"we're get the details of Sleipnir's conception. His father, Svadilfari, gets so hot and bothered when he sees Loki in the form of a beautiful mare that he breaks his restraints, and the two horses share a wild night in the woods.
Sleipnir's a living, breathing example of what Dr. Ian Malcolm says in Jurassic Park: "Life [read: sex] ... finds a way." (Check out a Family Guy spoof here.)
In Jurassic Park, the lives that find a way are monstrous man-eating dinosaurs (and some nice brontosauruses too). Sleipnir's not quite so scary, but he is sort of monstrous. His eight legs make him a mutant that's not easily categorized. This mutation probably symbolizes the character of his father, Loki, whose shape-shifting ways and position somewhere in-between god and giant make him difficult to define.
Mutant animals are often used to symbolize border-crossing and often able to help others make border-crossings that might otherwise be difficult. Sleipnir, for example, can carry his rider to the Underworld. Aragog, Hagrid's ginormous spider in Harry Potter and the Chamber of Secrets has children who hatch in the castle, leading Ron and Harry away from the safe world of Hogwarts into the dangerous, unknown world of the forbidden forest.
Mutant animals can also symbolize category transgressions (mixing of groups that don't usually mix). As much as we'd like to bring up Breaking Dawn's Renesmee as an example, what with her mutant name and non-standard parents, she's not exactly a magical animal, so we'll go back to Harry Potter. Lots of the animals Hagrid loves are mutants or combine the characteristics of several types of animals: the hippogriff is half-bird, half-horse, and his beloved dog Fluffy has three heads. Like Sleipnir, these animals have a father-figure who also defies easy categorization, for Hagrid himself is half-man, half-giant. As frightening as the mutants may be, Hagrid relates to their position in between worlds, and recognizes that this position sometimes grants the occupant unique powers.