As a sideline from the weird and wonderful characters, what are the weirdest and most bizarre creatures that you've devised or encountered in D&D?

I made a mimic bladderpus for my game. It was a mimic octopus which could imitate things, but was the size of a human and filled air sacs with gas to float in the air. It was a big hit!

Once, the final encounter in a game I ran back in 1998 was Michael Flatley. I ran him like a true villain, too.

Taking a bit from the other thread and mixing it in here, an encounter another time was an opposing party of adventurers designed as PCs, but run as monsters.

Poorly/paradoxically-defined creatures from Carroll's Alice stories. A DM I watch put them in the Feywild.

They finally defeated(?) an unbeatable bandersnatch by just turning and walking away from it after remembering the warning they received. ("...and shun, the frumious Bandersnatch!" from the poem "Jabberwocky" in Through the Looking-Glass.)

It was a literal "if you can't see it, it can't see you" situation. It seems none of them knew/remembered the Alice stories and never heard of any of the creatures that the DM added.

They had to (re?)tame and return a jabberwock pet to a Good-aligned Hag—a Hag who had been imprisoned by her three Evil-aligned Hag sisters. Luckily despite the weird description of the thing in the texts, the 1971 (and many subsequent) publications included a sketch of a young man with a large sword facing down a huge, very weird, and fierce-looking beast. The Bandersnatch has no description in the poem and no accompanying sketch, either.

A bit late to this one but...

The Cuckoo-trice (Tiny Aberration) is the result of a Cockatrice interbred a common Cuckoo and looks like a Tiny sized version of the standard Cockatrice, its eggs look almost exactly like common chicken eggs and it has a bizarre tendancy to lay its eggs in chicken coops.The call of the Cuckoo-trice is horrible, sounding somewhat similar to a normal cuckoo's call if said cuckoo was gargling some custard. It is theorised that this creature was the result of some Hag or other Fey influence

Eating a Cuckoo-trice egg, whether raw or cooked, forces the eater to make a DC11 Cons save, On a failed save, the creature begins to magically turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure , the creature is petrified for 24 hours.

The Cuckootrice is otherwise mostly harmless as, due to its Tiny size, its attack does only 1hp of piercing damage and it only has 13hp although some farmers have fallen foul of its petrifying peck.

Ooh, I just recently ran my 14th level party against a group of Giant Space Oysters! They're in Spelljammer territory and I wanted something appropriately strange. These things are category Huge, carnivorous pack hunters, with psionic powers to stun creatures while the Oyster scoops them in their "mouths". 

https://www.dndbeyond.com/monsters/3396689-giant-space-oyster

The above is a rough run through of their abilities, probably not balanced to the appropriate CR. It was a very fun fight for the party - they were in their Spelljammer firing cannons and trying to avoid a group of mercenary astral elves while battling two of these oysters. (I have a party of 8 PCs, so normal CR scaling tends to fail me)

Hm, I came up with the maggotaur once. Half maggot, half lamprey. All white, rubbery, slow-moving, relatively high AC but relatively low HP. They keen in what sounds like desperation as they slowly scuttle towards ant living prey on many short, stubby, half-useless legs. They're blind, but that's immaterial because they have 'blind sight' of sorts.

With a move of 20', they aren't really useful in any situation that allows them to be kited. But if they manage to close, and bite, they latch on, and drain blood, and gain color, and grow, and become stronger. Their rubbery white skin tears and peels as they grow.

After three rounds of draining blood, they become a Maggotaurus Rex instead, shedding what white skin remains, revealing the new, blood-red skin beneath - and small, colorful and pointless wings, too. The Maggotaurus Rex is not slow, or weak, and has arms, and eyes too. In this new form, it will seek to subdue as many victims as possible, laying eggs in them. It will then die of it's own accord.

The maggotaur aren't any sort of natural race - they're the 'offspring' of an undead thing who 'gives birth' to them. They sort of just drop out of her as she moves around. When they feed, part of their energy goes to her, making her stronger. In theory, her aim is to return to life so she can take care of her children (her backstory is as sordid as it might seem - it's also maybe too dark for this forum?!). 

Anyways, those are kinda weird and/or bizarre =)

So, this would be a two-part thing:

My first time GM’ing, I devised a two-shot where the party had to resolve a murder mystery at a harvest festival…the culprit had magically animated a scarecrow to carry out the murder; which on it’s own had managed to surprise the party.

Good encounter, good two-shot; and it even kickstarted a fully-fledged campaign (which I’m still very pleased with).

One of the amusing jokes that came about from that encounter, however, is that the party no longer trusted scarecrows.

Fast forward to my next campaign; the party once again finds themselves wandering some farmland…where they come across another scarecrow.

Amusingly, everyone at the table was wary of the scarecrow (despite their characters technically being different from before).

But one of the players decided: “What are the chances that he would recycle this evil scarecrow a second time?”

So he decided to inspect the scarecrow on its post…first he carefully touched it; then he whacked it a few times.

Nothing happened. Just a normal scarecrow.

…and then the earth shifted beneath his feet; and he got grappled.

I had planted a “Shambling Mound” directly beneath the scarecrow post…it was basically an angler fish that lured its prey using the scarecrow as the bait.

Everyone at the table lost it; they couldn’t believed that I’d fooled them twice.

I like to toy with weird concepts, and one of the oddest in the current campaign was a being who "twisted probability." I played it as all the d20 rolls were flipped. You had to miss in order to hit, and hit were misses. You had to fail saves to succeed, etc. So whatever the PCs were best at actually hindered them, and what they were normally bad became their benefited them. It got them thinking about their strategic options in a very different way by using their weaknesses to their advantage.

Story-wise it made sense with this being primarily needing to protect itself from very powerful attackers, so the more powerful and better trained you are, the more difficult he would be to defeat. Some random peasant would have a better chance of attacking, but for one thing, wouldn't be the kind of person looking for this being, also they wouldn't survive long against his attacks.

(To avoid the "I intentionally miss" option, I allowed them to intentionally fight poorly and roll with disadvantage because in my mind "intentionally missing" isn't trying to hit and failing, it is trying to hit right near them which would be unaffected. They needed to try to actually hit and fail for it to work but they could try to be sloppy about it.)

I have way too many to list.

Dog bone skeleton. A skeleton made of dog bones that gives buffs to canine creatures at its own expense. 

Chihuahua Cerberus. The most annoying dog multiplied by three.

Arms Dealer: A smuggler who deals in the illicit trade of arms. Actual severed arms.

I have an entire religion based around buckets and statblocks for its different followers.

There is the Grim Sweeper, sort-of-god of death and the Porpoise of Life who is the god of life. 

There is the four armed tyrannosaurus. Imagine how terrifying the beasts would be if they had four arms!

The Shuyu or shu-fish is a three headed, six legged, three tailed, red chicken that is mistaken for a lobster at first glance. They can be familiars. These are actual creatures from mythology.

Dingbats: fluffy guardians of nature who grab your ammunition before you can shoot at wildlife. These are from lumberjack lore.

The trust monster: easily mistaken for a rust monster, trust monsters cause your faith in your companions to erode.

Yearning Aloe Vera: an aloe that wanted to be an octopus so much that it practically got its wish. 

Got another Weird one to share!

Bitlings. These are my little creepy homebrew which my player hated, despite them being friendly!

They are formed magically and accidentally from the offcast scraps when a flesh golem is created. They stitch themselves together, and form an ant-like colony, with workers collecting body-parts to be sewn into new bitlings using the silk of the Queen, who has a "my children are the best" superiority complex to shame a private-schoolmum. The thread is a sought-after medicinal item, as it can sew body parts back together.

My player me Tim, the bitling worker who was working i nthe kitchen of the Thieves Guild. He was very enthusiastic to talk to her out of his 3 mismatched mouths, and was somewhat like an ovesized lobster that scuttled on mismatched fingers, with a scorpion-like tail with a human hand on the end. He made her sandwiches for when she left. Molly (the character) spent the entire time utterly terrified of Tim, who had a "wow you're an adventurer that's so cool can I show you my favourite toys" childlike vibe to him.

One of my more obscure creatures is the Speaker: A recurring antagonistic monster that has appeared twice now in one of my campaigns. Be warned, for this thing is kind of maybe sort of nightmare-inducing.

The Speaker is a 8-foot tall creature that has a humanoid torso and arms, though it has three spindly legs instead of two, and an elongated head with a split lower jaw. Its entire body is covered in bone-like plating and spines, protecting the soft red flesh underneath. Its appearance is unsettling enough, but the true terror comes in with its powers: The Speaker can telepathically communicate with creatures nearby, and has the unique ability to cast enchantment spells at its prey through said telepathy. It only hunts and eats humanoid creatures, as those are the only creatures it can use most of its spells on, adding to the creep factor.

Imagine the terror of suddenly hearing a voice in your mind, telling you to leave your house, or to open the door and let it in, and when you do so, suddenly seeing a towering monster lunge at you with jaws full of jagged teeth. Yeah, the party hates this guy, and they've stumbled across evidence of the Speaker chomping on people's craniums since the beginning of the campaign.

I had a type of ... corpse beetle. They share a certain trait with cranium rats: The more of them, the smarter they are.

Corpse beetles have their charming moniker because they eat corpses. They eat, lay eggs, multiply, eat some more. Eventually, they gain a group consciousness, and the ability to animate the skeleton they're stripping of sort bits. Also, they gain some of the memories of the dead body they're consuming. 

In a way, it's everyone's least favourite way of ressurrection.

Under the right circumstances, a colony of corpse beetles can slowly recreate an entire village - provided, of course, that they all died first. I've used them only once, in an orc settlement under a town in one of my campaigns. The orc village was razed by the humans topside, and left for centuries. When the PC's returned, the beetles had started reanimating the villagers, rebuilding homes and sowing crops (that they themselves would never eat). It was, initially, an eerie, ghostly place, with strange black-clad denizens wearing masks. Later, when the PC's had reason to return, the entire village was back, producing food and ore and weapons, and tentatively trading (via smugglers) with the topside village.

At the height of their 'power', the corpse beetles can destill the memories of their hosts into a potion they can offer others, if they so chose. This gives some interesting story options - all providing the PC's don't find them so repulsive they torch them all =)