Monsters that punch above their CR

[Older Beholder]

I was going to throw in Shambling Mounds, but they don't quite fit the punching up tag. Where they can cause fear among lower level characters, the tables turn pretty quick once someone learns 'Blight'

I ran a few sessions where Shambling mounds featured heavily in my game this year, perhaps it's no small coincidence that it was around the same time I was reading 'Day of the Triffids'

 

[Older Beholder]

Don't underestimate domestic cats! If they are bloodlusted for some reason, the XP budget for 4 level 4 characters would be:

Easy: 50 Cats.
Medium: 100 Cats.
Hard: 150 Cats.
Deadly: 200 Cats.

At 2HP per cat, that is more effective HP than an equivalent XP budget of Orcs. And enemy attacks often overkill cats, causing most of the parties damage to go to waste. If the cats are being controlled by someone clever, they can even use their 40 speed and 30 climb to fight like quicklings: just eat those attacks of opportunity and constantly hit and run, or drop from trees or the ceiling.

Oh, and nobody expects that first cat attack.

Player: I cast 'Swarm of Cats' on the Hag
DM: The Hag uses her 'crazy cat lady' ability by placing saucers of milk on the ground, making her immune to the swarm of cats...

 

[Al'Kelhar]

Oops, sorry, double post.

 

[Al'Kelhar]

As I think has been stated, lots of the mean monsters already called out have significant "save-or-suck" powers. But how about the boring old xorn?

AC 19
4 attacks at +6 for 3x claws 1d6+3 and bite 3d6+3
Can't be snuck up on (tremorsense), hides in plain sight (stone camouflage), and pops in and out of melee better than a teleporting fey (earth glide).

Better be willing to part with your load of gems when these guys come asking for 'em.

 

[S'mon]

Lizardmen and Hobgoblins are both nasty for CR 0.5. Hobs have low hp but great AC and damage with martial advantage; lizardmen have good hp and 2 attacks. CR 0.5 Thugs also nasty with high hp, 2 attacks & pack tactics.