Crawlers
Crawlers are hideous, slithering invertebrate creatures that resemble enormous earthworms or maggots. They can often be found burrowing in the rotting flesh of Barrow-wights. Adventurers who defeat such undead foes will sometimes find that the battle is not yet over when a maddened Crawler emerges from the body of a vanquished Wight. A sub-species of Crawlers is the Glow-worms, a breed of slithering, luminescent creatures native to Moria.
Note: Despite resembling the modern conception of a worm, Crawlers should not be confused with Worms, a term used for a breed of Dragon-kind.
Background
Crawlers do not appear in the work of J.R.R. Tolkien. They were created for The Lord of the Rings Online. Inspired by Tolkien's giant spiders, the game presumes that many insects and arachnids of Middle-earth are of enormous size.
A list of all known sightings: