It's explained under the alignment rules section that Neutrality can "also reflect the neutrality of those who came before Law and Chaos: the Old Ones, the great Cthulhu, and the empty Void, and the emptiness of the time before gods."
As for the level title, most of them are erratic like that. Surprisingly enough the Druid one is actually somewhat fitting if you go by the Greek/Roman sources with human sacrifice rather than D&D's nature protector.
I also recall Rob Kuntz (http://en.wikipedia.org/wiki/Robert_J._Kuntz
) having written a module called "Dark Druids" that had them worshipping "the Dark God" also known as Tharizdun, the Lovecraftian Greyhawk Deity.
I could see Cthulhu's worshippers having mundane animals as "unholy". While a Cleric with a nature goddess like Ildavir considering Mundane Animals as unholy is hilarious.
The entry on turning says "A cleric can attempt to turn away un-dead, demons, and devils. Some clerics of particular faiths may be able to turn away other kinds of creatures, depending on what powers their deities award them." while the entry for alignments has a lot of creature types. I wonder if they're meant to be examples of creatures you could have as unholy to a particular God of that alignment rather than something every Cleric of that alignment automatically gets?
Could just be a small oversight that came out of putting Cthulhu and Ildavir in the same alignment of course.