Will asked about the differences between the earlier AC9 Creature Catalog (green bordered cover) and the later RC era DMR2 Creature Catalog (orange cover).
A poster on Dragonsfoot had made a couple of pdf files with all the 'missing' monsters, but when I tried to find them I found threads on DF, but the files are no longer up to download.
So I went through the tables of contents of the two (I've only got AC9 on pdf), and here's hopefully the complete list:
Creatures in AC9 that didn't make it to DMR2:
Creatures in DMR2 that weren't in AC9:
I think that's all. Might have missed one or two, as the books have completely different layouts. AC9 is divided into sections by monster type, while DRM2 just lists everything alphabetically.
AC9's Table of Contents also has every monster listed multiple times if it's got multiple names, or multiple words (Grey Philosopher is listed as is, but also as Philosopher, Grey later). DMR2 also collects some of the independent listings of AC9 under groups, such as the prehistoric animals.
But at a glance, there's what's different between the two editions.