I have mentioned this many times before but for me AD&D 2nd Ed was synonymous with Ravenloft for me. For most of AD&D's heydays, I was at university, either as an undergrad, in grad school, or working on my first Ph.D. So both money and free reading times were limited. I focused my efforts on the campaign world that I enjoyed the most, though I did dabble a bit into Planescape.
While I bought the Monstrous Compendiums as I could, I made an effort to get the Ravenloft ones.
MC10 Monstrous Compendium Ravenloft Appendix
PDF 64 pages, Color cover art, black & white interior art, $4.99
I don't remember when exactly I bought this product the first time, but I do remember I was living in my first apartment after the dorms. I thought it was amazing and I could not wait to use some of these monsters. This product also expands on many of the monsters that had been briefly mentioned in other products, namely the Ravenloft boxed set and some early adventures.
This compendium appendix covers 55 monsters "Bastellus" to "Zombie Lord" and includes the "demi-human" vampires. Up to this point, I had argued that only humans could become vampires, but I guess the Demiplane of Dread is such that any race can become a vampire.
In addition to all the monsters, this book includes an "Encounters in Ravenloft" that is helpful for the different rules that monsters can follow here.
MC15 Monstrous Compendium Ravenloft Appendix II: Children of the Night (2e)
PDF 64 pages, Color cover art, black & white interior art, $4.99. Covers just 20 monsters from "Brain, Living" to "Vampyre."
This second compendium draws from many of the adventures and books published for Ravenloft at this point. It has similar monster types to the first one, but all of these monsters are unique NPCs. For example, the MC10 had the Ermordenung creature, this one has the specific entry for Nostalla Romaine. Some, like Desmond LaRouche, the Half-golem and Jacquelline Montarri, even get 4 pages of treatment each. This is part and parcel of the nature of monsters in Ravenloft, each and everyone has the potential to become a unique encounter and a specifically planned one. This is one of the reasons I really don't do "random monsters" anymore. In Ravenloft, there never should be a random encounter. Even "non-Ravenloft" creatures get a unique Ravenloft treatment like Althea (medusa) and Salizarr (a meazel).
This might make the utility of this book a little less than the others, it is a book of NPCs really, not just monsters. The advantages though are a way to show how nearly any monster can get the "Ravenlot" treatment and expand to something more than a collection of HP to be traded for XP.
Monstrous Compendium - Ravenloft Appendix III (2e)
PDF 128 pages, Color cover art, black & white interior art, $9.99
This is one of the first "bound" Monstrous Compendiums I ever bought. By this time TSR had learned that the three-ring binder experiment was over. So no attempt here is mad to keep up that pretense.
This book is larger, 128 pages, and takes on the trade dress of later (middle era) Ravenloft products. This one does feature a guide of what monsters from other Monstrous Compendiums are suitable for Ravenloft. Additionally, the "Climate/Terrain" section lists which Domain they are found in or even when they are found on other worlds.
This book covers 120 monsters from "Akikage" to "Zombie, Wolf." Some are repeats, but all are updated. We get newer versions of Flesh Golems and Strahd Zombies, and yet another version of the Baobhan Sith. Some more vampires (Drow and Drider) and a bunch of Liches.
Monstrous Compendium - Ravenloft Appendices I & II (2e)
PDF 128 pages, Color cover art, black & white interior art, $9.99
This product features the final Ravenloft trade dress and is one of the last Ravenloft products to be wholly TSR and not TST/Wizard of the Coast. Again, like the Ravenloft Appendix III, this is a 128 page book that first appeared as a softcover. The monsters are the same as Appendices I & II; even dividing them up into two sections of Part I Creatures of Dread and Part II: Children of the Night.
If your goal is to print out pages for your own Monstrous Compendiums, then the original MC10 and MC15 might be the better choice. If you are collecting the PDFs to have all the monsters then this product is the better bet.
I am a Ravenloft fan. So I have all four.
I also find quite a lot in these I can still use in my 5e games and in my OSR/Old-School games.