Kazakus' lore gives him in a lot of mystery by suggesting that he may not be all that he appears. Much of his background however, implies that he is actually a dragon.

The most blatant is his card text "The mysterious leader of the Kabal is NOT a dragon, and does NOT deal in illegal potions. Any public statements to the contrary will be met with litigation and Dragonfire Potions." The defensive nature of the text leads one to believe the opposite of the literal meaning and that Kazakus actually IS a dragon.

I've always found it weird that the ending scene in the Gadgetzan trailer with all the gangs doesn't feature Kazakus. Then I noticed that there's a huge dragon that's literally just Kazakus. Everything down to the tusks and beard. Picture in trailer: http://imgur.com/a/lHX5G

You might think that Kazakus would need to be tagged dragon on his card. This is confirmed not to be true by Wrathion who's description reads: "Wrathion, son of Deathwing, is a dragon. Why isn't he tagged as a dragon, you ask? WHAT, ARE YOU TRYING TO BLOW HIS COVER??" Wrathion is also humanoid like Kazakus and suggests that dragons tend to hide their true identity. The Gadgetzan Gazette Volume 32 #8 also mentions some rumors about "secret dragon people" infiltrating Gadgetzan in disguise. The Gazette post: http://us.battle.net/hearthstone/en/blog/20344072

I'm sure most people already thought that Kazakus was a dragon but I just wanted to put some of the evidence together to prove it.