No, the plural of sleipnir is not sleipnirs

Paizo does it again: they multiplied Sleipnir into an entire race of eight-legged horses (a la Pegasus in Greek myth). And, of course, they are typed "magical beast" instead of giant (or half-giant?) even though Norse myth did not make a distinction. To add insult to injury (the latter is the injury, not the former, since they falsely claim to be accurate to myth), they applied the English plural suffix -s to make the plural sleipnirs. As you can probably guess, this does not match Old Norse noun declensions at all. As far as my research could determine, Sleipnir was a name (derived from a root for "slip", "slipper", and "slippery", referring to him slipping between the nine realms) and never had a plural form. If I had to guess from other nouns with the same ending, the plural would probably be sleipnar. These sleipnar would then be the descendants of Sleipnir. Speaking of which, Sleipnir never had a surname, but it probably would have been Lokason or Lokison.