I've never seen all the monsters in the game. Sure I've seen all the zombie types, but not all the other stuff. I like that there is a sort of rarity to some of these creatures. Like the sludge crawler. I've seen maybe two sludge crawlers in the year or two I've played cataclysm. The first was in a sewer tunnel, and the second was in an open field being harassed by zombies. I remember I had a hard time actually damaging it.
looking through the list of monsters to see what else I've never met, I just came across the DOOM entries. The archvile is found under the name "vile." More interestingly is the creature called the "archdemon." It's the icon of sin XD. And just came across the marloss man entry. I assume that is a reference to the marloss man of cata legend and will be difficult to bring down. So many new monsters. I think some or most of these are down to the PK reimagining mod. Still, it's cool.