I snagged the new Monster Core for Pathfinder's 2E revised series, and have to say this is a nice, robust tome with a larger page count and content than the original. Like the new ORC licensed editions it expurgates as much OGL styled content as it can, which is to say that you won't find any distinctly D&D-brand level monsters anymore, not even the ones which served some general purpose. Owlbears - gone. Mimics? Gone. Darkmantles and other oddities such as the Chuul? Gone. Some of these monsters may be repurposed or renamed....ones which had already been given a distinct Pathfinder identity (Allgholthu, caligni, and xulgath for example being faceless ones, dark creepers and troglodytes) still lurk in these forms, but if you take a look at the section on demons and devils: whoo boy, this is where the identity of Pathfinder now feels more defined by what is absent than what is present.
Amidst this are the many monsters out of myth and folklore oddly untouched, even the poor Medusa who deserves to be properly renamed a Gorgon, but oh well.
I haven't decided yet if this is a good thing or not. Sure, the old Bestiaries work fine with the revised rules and provide those old OGL stats, but I'm a bit of a purist and want to look at the current state of the game for what it is trying to be going forward. Right now it is....interesting. A GM with this book will want to take some extra time to start looking for what will be the new oddities players come to associate with PF style fantasy, and what will be the useful staples for impromptu encounters. They are definitely not going to look like the old D&D stuff.
On the plus side, this actually makes the experience of reading the Monster Core feel a bit novel and different. There's effectively a new and different experience here, and the excision of so many OGL elements makes it feel more distinctly its own thing. On the negative side, it really does feel less like "something new and different" after a while and more like something defined by an absence, probably because ultimately Pathfinder really has always been the "Other D&D" for all practical purposes. What is a fantasy world (or Golarion) like without so many of these monsters that used to define it? What is everything in Pathfinder now, a game which was originally the de facto flagship for the D&D 3rd edition game system, now that it has been rendered neuter by corporate chicanery and a need to brand one's IP more distinctly? It feels a bit.....hollow, almost performative.
Despite that criticism, I am still keen to explore this book in depth and see what sort of ideas for a campaign it may bring me. We shall see! I have to reconcile my deep dislike of Pathfinder 2E as a player against my deep appreciation for it as a GM. I haven't quite managed to resolve this internal schism in my feelings about the game yet.