Giant Spiders (like Maexxna) still can't swim despite all the other spiders knowing how to.
Chromaggus can't swim either while his other core hound cousins (even the flaming ones!) can have a bath just fine.
Boss silithids have the same problem although it's kind of understandable for them since they are on a totally different skeleton, maybe making the mantid tanks can be the right occasion to teach the bugs how to swim?
I guess the beta could be a good time to point for these small problems and hope for some small tweaks, i know it's mostly cosmetic stuff but why ignore it anyway?

I'm sure there are other models out there which lack a full set of animations however these are those i checked to far, feel free to add more if you discover any!