We wanted to focus on more popular, mainstream monsters with the widest possible base of people interested in using them in campaigns.
Urdefhans were never seriously considered.
My position is that we need to have a book like this prove itself with much, much more common monsters than that before we can dream of diluting the customer pool with creatures they don't recognize.
Monster Codex 4? Sure.
Some sort of Pathfinder Campaign Setting Monster Codex? Absolutely.
Not when so many folks were skeptical of whether they would need this book, and not when we're trying something that has basically never been done before.
Emphasis mine, i.e. bold & italics added above.
So... Would this perhaps be a hint (spoiler?!) at one of the five "still unknown" monsters to be featured in the up-&-coming Inner Sea Monster Codex, hmm??
Of course, it could all be a hypothetical example on your part (& I'm just speculating ;p ), BUT...
:D :D :D
PS: You don't have to confirm or deny it, if it's a lil' too early to say, BUT...
<edit> Added post script & spoiler.