To my knowledge, there were four of these in total. I never owned the print copies, at this time I was getting married and moving into a new house, though I have been able to get the PDFs from DriveThruRPG. Curiously, Annual Vol. 2 has not made it to PDF yet.
Monstrous Compendium Annual - Volume 1
PDF 128 pages, Color cover art, color interior art, $9.99. 129 monsters, Aballin to Xaver.
This first book took on the trade-dress and style of the early AD&D 2nd Ed line and was a companion piece to the hardcover Monstrous Manual.
There are a lot of monsters here I have seen in later editions of the game and some are completely new to me. There are a surprising amount of dragons for example. There are few I recognize from 1st Ed that I guess had not made it over to 2nd ed yet (Gibbering Mouther as one example). There are a also a few I recognize from Ravenloft, given a more "generic" or general approach.
It is a good collection of monsters, to be honest. While the page are formatted to fit a book and not really a Monsterous Compendium (the left or right justification of the text on titles) you can still take this PDF and print your own page to fit into your Monstrous Compendiums. I am going to do this with the dragons for example.
Monstrous Compendium Annual - Volume IIIPDF 130 pages, Color cover art, color interior art, $4.95. 131 monsters, Alaghi to Zhentarim Spirit.
This third annual takes on the trade dress of the later printing AD&D 2nd material when the "2nd Edition" subtitle was removed. The formatting looks transitional. That is I see here the original Monstrous Compendiums eventually morphed into the style I associate with the last years of 2nd ed (and TSR for that matter).
The volume includes a lot of monsters I had seen in various Ravenloft and Forgotten Realms publications at the time and a few that I assume got their origins in the Dark Sun and Planescape product lines. There are some that also first appeared in the Creature Catalog from Dragon Magazine (Lillend for example).
There are few more dragons here too and, in a surprise, two demons / Tanar'ri. So something here for everyone.
This book also includes the Ondonti, the Lawful Good Orcs. So don't try to tell me that "Good" orcs are a new thing.
Monstrous Compendium Annual - Volume 4
PDF 98 pages, Color cover art, color interior art, $4.95. 104 monsters, Ammonite to Zombie, Mud.
This fourth and last Monstrous Compendium Annual was published in 1998 by Wizards of the Coast, though the TSR brand is still on the books. Additionally, this book also indicated where each monster came from whether Forgotten Realms or the pages of Dragon Magazine. There are some that I think are original to this volume. There is even a monster from Alternity here, which is a big surprise!
I would also like to point out that this is the first of these Annuals that acknowledges that it is based on the original D&D rules created by E. Gary Gygax and Dave Arneson.
There are quite a few new-to-me monsters here and few I have seen in other places before. It is nice to get them all into one place.
These annuals certainly represent the widest variety in monsters I have sen in any of the other compendiums. If I were to play AD&D 2nd Ed again, I think I would start with these as my sources for new and different sorts of creatures. I am sure that people that were still playing at this time (I had gone on an AD&D sabbatical from 1996/7 to 2000) might be more familiar with these books and these monsters, but it is a joy to open a book, even one 20-25 years old, and see something new.
I am now at the point if I print these out I am going to need a third 3-Ring binder.