The small flightless birds are in a brushy courtyard. They will lunge at the characters and try and latch onto them. When one latches on, another two burst out of the bushes. The fight is multiplicative, if you can remove birds two others rush to hide back in the bushes. They aren't terribly dangerous, and could provide many rations for the desperate adventurer.
2: Back to the Drawing Board
Upon entry into this room the party finds itself on a balcony overlooking a large maze. The maze is diagrammed on a board on the balcony with notes on traps and monsters. If they change what is drawn on the board the maze alters to match. (They should try the maze first, then drop some hints about the drawing board)
3: I'm All Thumbs
These are just a monster made of thumbs, like the thumb-thumbs from Spy Kids but even more horrible to look at. They can wield a bunch of different weapons at once, because they are OPPOSABILITY INCARNATE. The trick is that they are pretty short, have poor reach, and are rather clumsy.
4: Barking up the Wrong Tree
The room of the dungeon is open to the air with a grove a trees growing. There is a small man with a crossbow who, after taunting the party and shooting them, nimbly scampers up a tree. If pursued the party wont find him, and he'll shoot them from some other tree. He'll keep appearing in a different tree. The solution is to cut down all the trees.
5: Beating a Dead Horse
This thing just wont die. The party will defeat it, move on through the dungeon and it'll drag itself in still wanting to fight at the least opportune moment. Its like the black knight from Monty Python, but stretched out over a whole dungeon.
6: By the Skin of Your Teeth
You have to beat this monster by exactly its HP or it wont die. Its like the very end of parcheesi where you are waiting to roll the right number to go home. Forces you to adjust weapon load out to do less damage. Need to project this situation to the players, otherwise this fight will just piss everyone off.
7: Don't Count Your Chickens Before They Hatch
...or else they will be demon chickens. Found in a room with a bunch of nests and eggs about to hatch, and if one of the players asks how many there they will hatch as a lot of chicken monsters. Otherwise they just hatch as cute lil chickens.
8: Elephant in the Room
A behemoth ineffectively hiding in the corner of a room that wont attack unless you acknowledge it. Probably has a lamp shade over its head.
9: Get Bent Out of Shape
This is just how this monster attacks, it tries to bend you out of shape. Its magical so it wont break your bones, but it will bend them into horrible shapes and leave them that way. It has big strong hands.
10: Head in the Clouds
This monster keeps its conscious non-locally, specifically in the clouds. Its reaction times are pretty slow but it is protected against psychic attacks, and killing their remotely controlled bodies will annoy them and they will probably hunt you with their next ones.
11: Heard it Through the Grapevine
These magical grapevines run through the whole dungeon, whispering what they hear all over the place. Useful and dangerous for a party! Who knows who else is listening?
12: Hit the Sack
A big ol room with a bunch of big ol baddies. Suspended from the ceiling is a sack. If you hit it they all fall asleep.
13: Ignorance is Bliss
These poor guy have been in this dungeon for a long time, probably from kinder times in the past. They will ask about the outside world and the more you tell them the more upset they will get. They eventually fight you, but if you leave them ignorant they are happy and docile.
14: Let the Cat Out of the Bag
There is a bag that the party might find in some loot. Its decorated with embroidered kittens, and seems to have a live cat in it. If you let it out of the bag it becomes a massive cat monster and is quite upset, it will fight to get back in its bag.
15: Don't Look a Gift Horse in the Mouth
The dungeon's goblins are giving the horse as a peace offering...so they say. Don't look it in the mouth or you will start to notice that its a horrible horse construct (like Arnold K's fucked up horse monsters). Once they realize they are compromised the goblins operating it from inside will cause it to self destruct. Could be weaponized if given to other unsuspecting people.
16: Pull my leg
A giant crab monster who's legs are really easy to rip off if pulled, otherwise quite dangerous.
17: Shoot the Breeze
A shootable but antagonistic wind that harasses the party. When they try and climb something the breeze is there to blow them down, it'll blow dust in their eyes when they are fighting, and just make a general ass of itself. It also really likes to talk.
18: Straw that Broke the Camel's Back
This talkative dried stalk of grass has grown too proud of its defeat of a camel, it'll try and get someone to put it on your back. If it succeeds it breaks your back.
19: Two Birds with One Stone
These huge carnivorous bird must be killed in the same round or else they wont die. The room they are caged in has a few mounted ballista that pass through foes.
20: Devils Advocate
There is a devil in this room with a normal guy. The devil will engage the party while the guy says things like "You know, [the devil] really isn't that bad", "I know its an unpopular opion, but maybe it [the devil] is right", or "Okay, okay. For the sake of argument you folks look like the bad guys here".
Thoughts:
Most of these are bad puns, but it could be fun to populate an idiom dungeon with these if your players are down with a slapstick dungeon. They are really easy to write, so share yours below!