Lamest Creature You've used to good effect in your campaign

[lukelightning]

Unicorns. I've changed 'em from goody-goody fairy princess ponies to murderous sociopaths that love to impale things on their horns and let the corpses "ripen" (as they only like to eat spoiled meat). However they are protected by a powerful glamour that charms people into thinking them beautiful and harmless (this is in lieu of a magic circle against evil ability).

 

[el-remmen]

Flail Snail w/ fire elemental template to represent weird mutated creatures slipping between tears in the veils that separate the planes.

 

[frankthedm]

I've never considered displacer beasts in the least bit lame.

OP did include absurd, IMHO DB are a touch 'out there'.

 

[SpiderMonkey]

I used Displacer beasts as extra dimensional shadowy feline creatures. They had better feats [combat reflexes rather than alertness] and Darkvision but suffered Light sensitivity and disintegration side effects [0 or fewer HP=Gone in a foul smelling Poof!]from lightning and light based attack spells.

I added the fire element template to stirges and described them as flamining vampire bats. [birthed from destroying a particular vampire strain by fire.]

Me likey!

 

[Baron Opal]

I've used gelatinous cubes as doors in one abandoned temple that was taken over by an oozemaster. My biggest success was a small colony of triapheg that were the dross of gods creating life. Basically, what was in the wastebin woke up, and self assembled.

 

[Huw]

My biggest success was a small colony of triapheg that were the dross of gods creating life.

Ooh, nice idea. I'll have to steal it

Got any uses for an imorph?

 

[Raven Crowking]

Invented monster...funny & lame

In 1e, I once had the PCs searching a place that had been inhabited by a wizard. He had kept both potions and root vegetables in one pantry, and under some smashed potion bottles, the weird admixture of the potions had brought life, sentience, and "humanoid" form to the carrots that lay in a burlap bag.......

......The tiny humanoid carrots then claimed and defended that section of the building.

Not a huge combat challenge, but it was fun.

 

[Shemeska]

Gelatenous Cubes. I've had a few prize moments using those guys as props in some particularly dirty tricks, and they work so well precisely because they're so cliche that they're cool.

 

[Voadam]

I was in a game where my good merchant prince wizard was visiting a drowesti elf (combination drow and melnibonean) diplomat who hosted him and his party. In addition to giving everyone in the party powerful valuable gifts the ambassador served me rare melnibonean delicacies. I ate the fried flumph with relish, having no idea in character what a flumph was.

 

[Agent Oracle]

I scared my party witless with a single choker.

They were level 3, it was a CR 2 encounter, fair game right?

The thing was, there was no dedicated healer in the party. We had a dragon shaman: which means: "fast healing 1" within 20 feet (but only to half hit points) and the party had taken it pretty hard on the chin yesterday, so they were all low on health to begin with...

I I had a choker waiting down the hall, something to give them a scare and a good combat. Ran it as-is from the Monster manual, including it's proclivity towards hiding up high and grappling with it's 15' reach... Except, they never saw the whole thing. It got a surprise attack against the rogue (who was leading the party looking for traps) and grappled him, then pulled him up into a side tunnel and gave him a squeeze. I one-hit killed the half-hp rogue, and scared the rest of the players into retreating from the tunnel.

I didn't mean to do it, honest!

Ever since then, the party's been terrified of things with tentacles. and this isn't even call of Cthulhu!