Grodbog
Gredbyg (singular: Grodbog) are a breed of large insects most commonly found in the deep underground tunnels of Moria, Gundabad, and other cavernous regions of Middle-earth. Their life cycle is marked by a dramatic transformation: Grodbog hatchlings resemble beetles or pill bugs and are relatively small, usually non-aggressive unless disturbed, while fully grown Gredbyg are towering, mantis-like predators encased in thick chitinous exoskeletons.
In the long absence of the Dwarves from Khazad-dum, the Gredbyg swarmed through the abandoned tunnels of Moria, establishing hives and overrunning entire sections of the ancient city. These mature Gredbyg are fiercely territorial and will attack adventurers without hesitation if they sense a threat to the hive. They have since become a serious danger to the Iron Garrison and other Dwarven efforts to reclaim their ancestral halls.
During the War of the Ring, sightings of Gredbyg increased beyond the deep places, suggesting a wider spread of the species. Colonies have been reported along the Great River and within the Under-wall tunnels of Rohan, indicating that these creatures are capable of adapting to new underground environments far from their original lairs.
Background
Gredbyg do not appear in the work of J.R.R. Tolkien. They were created for The Lord of the Rings Online. As with Tolkien's giant spiders, the game assumes that many insects and arachnids of Middle-earth exist in monstrous proportions.
A list of all known sightings: