My wife and I watched the D&D movie yesterday. Don't worry - I'm not including any spoilers here. But she and I had a discussion of some of the "iconic" monsters she hasn't encountered since starting the hobby 7 years ago, and maybe why that is.
My thought is that many of the classic creatures from her list harken back to an old style of play that has disappeared amongst 5e players (which is when she started the hobby).
Here's a selection from the list we made this morning (and the reason why I think she hasn't encountered them) ...
D&D isn't in Dungeons anymore...
Black Pudding
Gelatinous Cube
Carrion Crawler
Mind Flayer
Intellect Devourer
Roper
Umber Hulk
I think "dungeon" has just expanded beyond what OD&D called a dungeon. A dungeon isn't a series of underground monster-filled rooms connected by trap-filled corridors. A dungeon is a series of connected encounters. The dungeon could be anywhere.
That said, I know I've used 5e Mind Flayer, Roper, Umber Hulk, Carrion Crawler and Gel Cube. I can't remember the last time we used a Black Pudding, but it was probably 3e. I'm quite confident we've
never used an Intellect Devourer in any edition of the game, but that might have to do with our suitability as a food source for such a beast.
But I'm pretty certain some of those were in official modules.
D&D doesn't like to trick players anymore...
Mimic
Roper
Gelatinous Cube
Rust Monster
We've used Ropers, Gel Cubes, Piercers, and Mimics. Mimics are pretty rare, though. The first three I'm pretty sure were in official modules, even, but I could be mistaken.
I don't think we've ever used a Rust Monster. The last one I recall was in a 1e/2e campaign in high school. It's kind of an intentionally unfair monster. Kind of like Black Pudding. We don't really care for stuff that destroys equipment, I guess.
Mid-Range CR creatures have limited windows of use...
(You can't use them like goblins in large numbers, and a single one isn't a challenge. They don't really have a place in encounter design.)
Displacer Beast
Intellect Devourer
Carrion Crawler
I guess I haven't noticed this problem, but aren't these all CR 2 or so? I use groups of CR 2s all the time -- Ogre, Zombie Ogre, Chromatic Guard Drakes, Ankheg, Owlbear, and Veteran are some of my go-tos -- so I guess I'm confused. They're a great distraction in mid-level combat. Still threatening but rarely do much damage. They make great monsters for encounters with deceptively difficult terrain.
By the time you get to that level, the campaign will end or else the monster won't be a challenge...
Beholder
Marilith
Tarrasque
I've never used the Tarrasque. Godzilla is boring. Well, I think in 3e we did encounter it once, but it was kind of like a post-campaign super boss for fun after the campaign wrapped up.
I've definitely run Beholders and one of our players loves to run Mariliths so we've fought them before.
I have noticed that some DMs are hesitant about throwing high CR creatures against the PCs. IMX, CR stops really mattering around level 11. We had our party of 5 level 13 PCs take out a Balor. And then a minute later take out a second Balor. It nearly killed half the party, but nobody actually died. They just got knocked unconscious. That was in Out of the Abyss.
Edit: Clarity.