Aw man, can't I just skip to the Ukss contribution already? I guess that's one of the perils of reading these books whose whole deal is just being a list of ostensibly interesting things, presented without commentary - sometimes they're right about being interesting, and that makes commentary much more difficult.
Giant owl-folk? Intelligent clouds who know deep lore about the multiverse, but are tough to communicate with, because they are actual, up-in-the-sky, miles long mountains of mist? Cursed crows that follow you around, afflicting you with a mysterious illness as they feed off your life force? Whatever the fuck a Hollyphant is (some kind of miniature, winged woolly mammoth whose trumpet can exorcise demons)?
Yes, thank you. These are all relevant to my interests. I could go on. . . so I will.
The guardinals are pretty cool. Though it's little weird that the ultimate champions of pure good are also AD&D's most straightforward furry races. Not to impugn the moral rectitude of the furry community or anything, it just seems to me like Leonals (lion folk), Equinals (horse folk), Lupinals (wolf folk), et al would be great candidates for an anthro-fantasy-centered campaign setting, and it's weird that we're seeing this concept for the first time by making them the social equivalent of the angels.
But then, genre was always D&D's Achilles Heel. It flirts with it, and sometimes even manages to do some pretty interesting things (it's no coincidence that most of my favorite entries from this particular book are the ones that evoke fairy tales or borderline-sci-fi weird fantasy, like the straigt-outta-Dark City race known as the Keepers). However, it always tries to keep at least one foot in its comfort zone. Even Dark Sun still has elves and dwarfs.
So you look at this conservatism, and it's sometimes a little confusing. "Fantasy world where all the intelligent species are human-animal hybrids" is a dead simple pitch with obvious appeal, and your multiverse-spanning portal fantasy is the perfect place to test the waters and maybe sneak it in through the back door, but AD&D decides that the animal folk will be the celestial manifestations of Neutral Good, to match the Lawful Evil Baatezu, the Chaotic Evil Tanar'ri, the Neutral Evil Yugoloths, the Lawful Good Archons, the Lawful Neutral Modrons, the Chaotic Neutral Slaad, and the newly introduced Chaotic Good Eladrin and True Neutral Rilmani.
I'll admit, there's a certain charm in the grid filling, but sometimes I wish AD&D could just use its ideas as ideas. There's some good stuff here, but it's weakened by trying to fit it into such a rigid structure.
Like, seriously, what is up with the Rilmani. They definitely needed another pass, because it's clear that they were designed towards the needs of the structure and don't have much identity beyond it (actually, the guardinals are the same way - of the three new celestial supercategories, only eladrin can really stand on their own). They are a collection of True Neutral species, with each phenotype filling its own mechanical and narrative niche, just like the numerous types of demons, devils, and angels. But the niches they serve are all variations of D&D's worst alignment.
The thing is, they're not, on the surface, a poorly designed group of creatures. They have a very interesting visual style (mostly thanks to DiTerlizzi, who also did 100% of this book's artwork and continues to be the setting's MVP). And their role in the story isn't too bad either - they've got this weird alien imperialism thing going on where they travel to different worlds and impose their ideology on the locals with means that vary between military force (the ferrumachs), assassination (the cuprimachs), to sending advisors/agitators (the argenachs).
The general impression I got was of an imported space opera trope - the serene elder aliens who take it for granted that their superior technology gives them the right to meddle and whose abstruse philosophical creed is so mysterious to the younger peoples that it can barely be parsed as morality. This is an impression that is furthered by the fact that their various sub-species really feel more like character classes or social strata than they do like distinct physiological adaptations (there's no mistaking a succubus for a balor, but the plumachs and the aurumachs definitely look like cousins). They're a group of people who may be tough for the PCs to relate to, but they are also a civilization with its own unique values.
And from a high enough level of abstraction, it works great. "Diplomatic observer in the inner planes, empowered by their home government to make policy on the spot" is an amazing idea for either an antagonist or an ally (or both!). Except that the rilmani are not mysterious elder aliens. We actually can understand their civilization's values . . . and they're fucking ridiculous. Make sure there's an equal amount of good and evil in the universe? Okay, buddy, whatever you say.
Believe it or not, though, the last four paragraphs were praise. Everything that's uniquely Planescape about the rilmani is good. The one thing about them that they inherited from mainline AD&D is not. If I seem especially hard on them, it's because they're C+/B- work and the rest of the book proves beyond a doubt that this team is capable of making A's.
I'd go so far as to say that Monstrous Compendium Appendix II is the very best Planescape setting book to date, and I'm including the boxed sets in this assessment. I think it's because it shares a certain quality with the Sigil and Faction books (hitherto the best Planescape books) - it feels like it was written for Planescape. The first Monstrous Compendium had some good stuff, but it was a compilation. The boxed sets had a certain boldness to the design, but could never quite stop being the Forgotten Realms' afterlife. The entries in this book were dense with lore and demonstrated real thought being put into things like the food chains of the lower planes, the migration of intelligent species from non-PHB-compatible fantasy worlds, and the logistics of interplanar travel. It's a book that's specific enough to potentially be alienating, and it makes the world feel more lived-in as a result. My hope is that this represents the series turning a corner, and that it will become more its own thing as time goes on.
Ukss Contribution: Finally, we made it, and I used only two of my top three picks as examples in the main body of the post. My final choice is actually an example of a tendency in the book that slightly annoyed me - making creatures that are little more than "planar" versions of already existing Prime Material species. There's absolutely no call for drawing a distinction between Neogi (the anthropophagic, slave-keeping spider pirates of Spelljammer) and the Tso (anthropophagic, slave-keeping spider pirates, but on the planes). Neogi are good villains and it's a fruitful idea to have a population of them in the outer planes, but it's like having separate entries for Prime humans and Planar humans. It arguably detracts from the appeal of the game to say that they're too different.
However, my Ukss choice is another example of this - the khaasta. They're lizard people, but on the planes. I think they wind up justifying their existence by having a distinct culture (cutthroat mercenaries and raiders who wander from score to score) and some great art (naturally), but my reason for picking them is much sillier than that. . .
They ride giant lizards. Lizard-folk who ride lizards! Something about this idea appeals to me on a fundamental level.