Demons also may not automatically be antagonists if your player characters are infernalists!
I would suggest as a start somebody (ie. not me...) compiles a list of the available non-human character stat blocks in the available official ArM5 Ars supplements to date as a start.
The current fan-compiled appendices for Virtues & Flaws (Erik Dahl), Spell Guidelines (Chris Allen) and Shape & Material bonuses (Erik Tyrrell) are quite useful IMO. Particularly when you are writing for SubRosa or the line.
Some nut even compiled a list of all the books statted out in official supplements to date... some people, eh? 
Supplement, page number, Might score and Realm (or mundane if no Might score) would be the minimum data set IMO. Shouldn't take too long. Excel file, print to PDF and hey presto!
You could certainly list SubRosa creatures in this - most have been written by line authors IIRC (and may potentially be recycled into the line in later supplements).
This index would be quite useful to see what is covered already (quite a lot if you count the Book of Mundane Beasts, the RoP books and some of the Tribunal books - tCatC has a lot of critters in it when I last checked and it looks like most made it in according to the preview ToC). I think you'd be surprised about how many ArM5 stat blocks there are already - for instance there are 4 new animals (all useful for shapechanging magi or as opponents for grogs etc) in Timothy's Alexander of Jerbiton section in Magi of Hermes (Bat, Crocodile, Camel, Lion) that you might miss if you didn't know where to look. OK the lion is a bit of a reprint from Mundane Beasts, but it would have helped to know this before I came up with a version for tCatC almost simultaneously (they were pretty similar actually, so I just tweaked Timothy's version to the Bactrian variant).
A compilation of existing work does not necessarily fit the fiscal priorities of Atlas or fans, but David might consider such a "Bestiary: Revised Redux" if it were pitched with additional new content that would make it valuable as a stand-alone supplement ie. a collation of all the beasts suitable as Heartshapes, add in some of the sections on shapeshifting, some pointers on designing mundane and Realm creatures within the rules and a whole swag of Story Seeds. Something a bit different than the previous version, perhaps more along the lines of the briefly mentioned Grogs book but for Beasties.
As a resource it would also help Storyguides and Troupes play/design sessions and when writing (for SubRosa etc) it would be damn handy. I can never remember where all those critters are... 
Lachie
All four od these are in RoP:F, though,,,
You're right on the hydra-- I was confusing them with hydros.
Consider it added.
hydra
kraken
troll
vampire
And I'll very much look at the index Lachlan is suggesting.
-Ben.
Ben,
I would love to see this tome assorted by region, if possible. Or at least a final list (download) with sortable columns, including location.
For my TARTALO creature in issue 1, I did a general google search for 'Basque Legendary Creatures' or something, and found one great link. (I was putting together an adventure in the Pyrenees). This inspired the creature / encounter I wrote for you. Of course I can't find that link now. If the link would be helpful, let me know and I'll try to dig it up at home.
But this is pretty good:
en.wikipedia.org/wiki/Basque_mythology
This seems to have good info on where the Basque creatures were found:
nabasque.org/NABO/basque_legends.htm
Can't wait to see the final results of the bestiary!
eric
There has been many good replies here. I think at least that the idea of a good bestiary has been stated clearly and gotten a fair treatment.
Timothy: Thanks for the tip, I will check it out when I get home to my books. (You really should publish RoP: F as an PDF soon)
Jarkman: Even though such an index as suggested above is a good start, I do think that we need an actual book collecting them all, and that one shouldn't be afraid of printing something twice, or expand or improve on monsters already sketched out (or to have several versions for different realms) . It would be nice-indeed with one book of high quality for both game mechanics, creativity, "in-game-feel" and usefulness as an side-arm for the storyguide and for quick and dirty fun.
The problem with a fan-effort is of course to get a decent level of quality to it. It takes a lot of time to create and think through good and flexible material.
Ben: I love the idea of the 72 demons, and using the pictures that you linked to. Regarding the Infernal Realms, personally I think that it has a lot going on for itself in terms of potential and nastiness, as it is displayed in the "RoP: I"-book and in the game as an whole. But it is still somewhat challenging to actually create demons that have both the right feel of corruption and danger to them. At least one have to use a lot of wicked creativity to get them right. I am quite satisfied with some of the ways that I have used demons to annoy and corrupt different players in different sagas over the years. In most of the cases they have never figured out about the demonic involvement.
I recommend everyone who need inspiration in how to use demons to read through the SubRosa #7-article : The Storyguide’s Handbook: The Enemy Within. It took a quite refreshing approach. I don't know how demons would work in a hack'n slash type of scenario/dungeon crawl, though....
One of the challenges is how easy it is for magi with the right Perdo-Vim might-strippers and strategic ward usage can obliterate demons, but this is a challenge for all of the different realms, at least in very high-power sagas.
Anyways, I will at least let a demon-creation effort rest as a possibility in the back of my mind.
I'm not Atlas, Night Owl. I just freelance for them quite a bit. 8)
About this I'm sure I've seen historical things from the Lesser Key of Solomon which are similar and in public domain since the illustrators have been dead for hundreds of years. Worth following up, possibly...