I've had the DMR2 version of the Creature Catalog for quite some time. I picked it up sometime while I was in college, so between '93 (when it was published) and '96 (when I graduated). I've gotten a lot of good use out of the book, as there are quite a few monsters that aren't well known by every gamer on the planet (unlike everything in, say any version of the Basic and either of the Expert sets). The monsters were primarily taken from modules produced for Classic D&D, but there are apparently a few originals as well.
A few of my favorite Creature Catalog beasties include:
Aranea (originally from X1 Isle of Dread): creepy, giant spell-casting spiders. What's not to like? And they were cool enough that the 3E designers decided to throw them in the 3E MM, so they're open game content now, too!
Deep Glaurant (this one may be original, I don't remember seeing them in any modules): big, scaly, demon-like underground predators.
Undead Dragon (again, not sure what module this is from, if any): Having read Pillars of Pentegarn and played plenty of Castlevania as a kid, I've always liked throwing in skeletal/zombified dragons in my games from time to time.
Gray Philosopher (pretty sure it's original to the CC): An undead cleric who doesn't do anything but ponder Cthulian mysteries while his evil animated thoughts attack you. Creepy and cool at the same time.
Nagpa (from X4/X5 Desert Nomads modules, forget exactly which one at the moment): Skeksis (do I need to say more?)
Tabi (not sure where they're from either): Flying Monkeys (again, kinda speaks for itself).
Wyrd (B7 Rahasia): Undead elves who attack by throwing glowing orbs of energy, similar to the way Venger attacks in the D&D Cartoon.