The "Fantastic Beasts and Where to Find Them" Comic Relief book has an interesting bit of information that ties in with this chapter. In the entry on Kappas, is says they are Japanese water-demons, not Mongolian despite what Snape says in this chapter. J.K. brings the reader's attention to this in the CR book by having "Harry" (or possibly Ron) write "Looks like Snape hasn't read this" (not an exact quote) in the margin, which would be strange if she had merely made a mistake in PoA.
Perhaps this means that Snape isn't as good at DADA as we've been led to believe?