In the Qadira book is the race you are looking for.
Where can I find this book?
In the Qadira book is the race you are looking for.
Ok, all of the above was very informative and all well and good but what about the dinosaurs?
How are the monster advancement rules? Is there any more elegant method for advancing monsters that rely on supernatural or spell-like-abilities, rather than just piling hit dice on and making them physically tougher, or giving them class levels which may not be closely related to their 'schtick'?
...We basically picked and chose demon lords from a mix of open content (thanks, Tome of Horrors!)...
LA/ECL is not a part of the Bestiary; this isn't a book of alternate player races. It's a monster book. That said, there is the aforementioned half-page or so of advice in the appendix where we talk a little bit about what to watch out for if you let a player play an ogre or minotaur or something like that. It's certainly possible... but it's not what the game was designed to support. Some day we might do a "Savage Species" style book that looks into the "play monsters as PCs" stuff... but that book isn't this Bestiary.
Note that Asura's a name we've already claimed for a race of outsiders—a race of lawful evil outsiders based heavily on Indian myth. We've only statted up one Asura so far (in Pathfinder #9), but another one's on the way at some point in Council of Thieves.
Both "Agathion" and "Azata" are based on real world mythos, though. Agathion is a Greek animal spirit, so it seemed logical to make the NG animal-headed outsider race agathions (and having the 1st edition agathion itself be a NG creature from the outer planes is a nice bonus there). The azata is a bit more obscure; the original spelling is in fact "Yazata" and they're from a Zoroastrian concept (check out the Yazata entry in wikipedia for more info).
So while both "guardinal" and "eladrin" are merely made-up words, the names for our equivalents in the PRPG are based on real-world myth and sources. This is a case, I think, where not being able to use the names that WotC claimed as their IP ended up spurring us to come up with even better names, personally!