Welcome to post 600.
I picked up the pdf of Pathfinder Bestiary 3 last week. I had heard it had some Lovecraftian creatures in it, and I was curious, but in general, I like mining monster manuals for ideas. Paizo's previous entries in the Bestiary series have been pretty good in this regard.
First off, there are a lot of mythological creatures repurposed in tried and true rpg fashion. Quite a few of these are of Asian derivation making this suitable for a "Oriental Adventures" sort of game. There are also creatures from the myths of Native Americans, Inuit, and Pacific Islanders as well. The Fiend Folio and Filipino folklore veteran, the Berbalang, makes an appearance.
There are a lot of other Fiend Folio also-rans. The dire corby, adherer and the flumph get entries, for reasons beyond my understanding. There are some Monster Manual II refugees too.
There are the obligatory expansions to giants, dragons, demons and devils. As is typical, the ranks of evil classes of creatures get expanded with the divs (evil genies) and asuras (philosophic devil sorts), and our old friends the demodands (who all look much more militant and badass in their illustration than the MM2 originals).
One of the things I like is the cryptid and more modern folklore entries. There's the hodak and globster from North America and the kongamato, lukwata and popobala (which was changed for some reason from popobawa) from Africa.
The aforementioned Lovecraftian critters include the moon-beast, voonith, and Yithians. There are other literary borrowings including the bandersnatch and the jubjub bird from Lewis Carroll and monsters likely inspired by other media: the hungry fog and the sargassum fiend.
There are a lot of original monsters, of course. Some of these (like the bogeyman and the pale stranger) are interesting, but seem better suited to a non-Medieval game. Then there's the cold rider, who's sort of a frosty Nazgul astride a demonic reindeer, and the deathweb--the husk of a giant spider animated by thousands of little spiders! Both of these guys would make cool one shots, at least.
Overall, I think it's a decent selection of monsters. More time is spent on more of particular, familiar clades of creatures than I would like (more giants, demons, devils, and variant dragons and dragon-like creatures), but I really like Paizo keeping alive the tendency to borrow entries from literature and modern folklore in addition to mythology.
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