Underdark wildlife

[Yora]

I am working for an underworld for a setting that is more like Khyber from Eberron than a more common kind of Underdark from Greyhawk or Forgotten Realms. That is, it's not just large caves underground, but almost like a different plane, or a second layer of the material plane, in D&D terms.
It's a world that has seen no real changes in environment since the planet was formed, which means the only force of evolution at work there is the fight for food. With no changes in climate and landscape ovef billions of years, the creatures living there are almost all ancient species from the beginning of life in the world. (Plus grimlocks, derro, skum, and driders, who are mutant descendants of humanoid explorers.)

Now I got a couple of creatures that live down there, but most are powerful highly intelligent aberrations who form the top of the food chain. But for an ecosystem, there also need to be lots of other creatures living there.
Umber hulks, carrion crawlers, giant spiders, and giant centipedes, as well as various oozes are pretty much all I have so far.
Any ideas what other creatures I could use to populate this Underworld? Pretty much all sources or new ideas are welcome. Converting them into stats for my campaign is easily done.

[jedipotter]

D&D is full of Underdark creatures. Check out books like Underdark, for example.

D&D has plenty of 'fungi monsters' to make the bottom of a food chain like shrikers, voilet fungi, phantom fungi, and such. Plus oozes and vermin. Lots of bats and lizards, of all types.

And the peicer....that looks like a stalagtite and stabs foes....

[Tanuki Tales]

I'm going to advocate Myconids, if only because I love the guys.

[JusticeZero]

I had proposed "Mold Forest" before. This is where you have a cave system into which something like sewage or a swamp slowly drains. At the bedrock level is a marsh full of fungus which blooms out to dig into the edges further, full of fungus creatures, with another kind of fungus or other effect draining the water excess.

[Silverbit]

I recommend looking at real-world stuff, and then reskinning it for an underground setting. Especially deep sea things. Imagine a land dwelling version of this monstrosity (the pelican eel). A long, thin, whiplike entity with a collection of eldritch glowing tentacles at one end and a massive fanged mouth at the other. It lurks in the shadows, consuming small animals and unaware explorers. Or a subterranean creature based on the pram bug. They hide within the hollowed-out bodies of other underdark creatures, then send out their loathsome young to capture yet more bodies.
They even look like fantasy underdark monsters:

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Warning: slightly scary translucent crustacean.

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Well, hope this helps you. Now I'm scared of my own aquarium.

[Scow2]

Take all the above-ground creatures, change the name to "Deep/Dark [original name]" of all animals, give them Darkvision, and turn them monochrome. Take all plants and name them "[plant name] Mushroom/Fungus". BAM! Instant underdark Ecology.

I was going to quote the relevant Vaarsuvius rant on the subject, but that's the entire dialogue of the copyrighted comic :(

[Zahhak]

Are you looking for a full sized, functional ecosystem? You could make an effective ecosystem based in an Underdark environment if you have the plant life be chemosynthetic. Basically, instead of using sunlight as an energy source for the production of organic material you would use inorganic molecules like hydrogen sulfide. So, you wouldn't have massive plant life, but you would have forests built around volcanic areas, with smaller plant life extending outward.

[Trekkin]

If you need a primary producer, I guess you could make some kind of arcanotrophic cyanobacterium for magic lichen. Depending on how it works, you could then have convergences of ley lines act like hydrothermal vents. I'm not sure there's another energy source rich enough to fuel an adventurer-worthy ecosystem.

[awa]

i second the idea of thermal vents producing providing the initial energy

Since you have a lot of aberrations you could include a creepy organism that resembles flesh that grows to cover the walls and floor pulling out nutrients and heat from thermal vents. Of course it bleeds when cut. It also absorbs any organic material left in contact with it the absorption is not fast enough deal damge immediately but trying to walk very far over it mat result in lost shoes and eventual damge.