Personally, after having seen Phoenixes, I would love to see bats in the game. I can't imagine anything looking cooler with my Necromancer. They could always increase the size of the model too, make it a dire bat or similar and having it having upside down off the character's arm when not fighting

The other pet I think that would look great in the game is a crow, for much the same reason above. Gothic ambience. Just as I feel a hunting hawk or similar would work well with a warrior.
As we're not likely to be seeing any pet gryphons or dragons, thought I'd offer up a couple of realistic suggestions. Anyone have some of their own?