Gredbyg
Gredbyg (singular: Grodbog) are a breed of large insects found in the depths of Moria, Gundabad and other places in Middle Earth, usually in underground cave structures. Grodbog hatchlings resemble beetles or pill bugs. They are small and usually not aggressive unless provoked. The full-grown gredbyg, however, are larger, mantis-like, and covered in a chitinous exoskeleton. They will aggressively attack adventurers that they perceive as a threat to the hive. In the Dwarves' absence, the gredbyg overran the tunnels of Moria and are now proving to be a danger to the Iron Garrison attempting to reclaim Khazad-dum. During the War of the Ring, the gredbyg have begun to spread to other regions as well, such as the Great River and the Under-wall tunnels of Rohan.
Background
Gredbyg 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: