Like the player races, some of the monsters have been leaked:
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So, from what we haven't seen already previewed, I'm very happy to see the babau (so the book isn't shying away from the main extraplanar races like demons), the bodak, the brontosaurus (YES! More dinosaurs! The lack of a typical sauropod from the MM was very noticeable, and hopefully we'll see a smaller predictor like deinonychus and/or velociraptor and a stegosaur at the least as well), and the catoblepas (which I had very much wanted to see updated). With what has already been previewed, and now knowing that we'll be seeing additional demons, dinosaurs, and the catoblepas, I can only hope we get gibberlings and quicklings in the book, and I'll be very near 100% satisfied with the updates!
It looks like this is in strict alphabetical order, and not in the order they will appear in the book, since we know the hags and the alhoon will be under "H" and "M" respectively in the actual book. I assume all the NPC-types will be in their own appendix (and there looks to be an interesting assortment of them, even in the first few letters of the alphabet). Those "Booyahg" types are goblin variants alluded to on the preview page we've seen already...