Source: Pathfinder 11: Skeletons of Scarwall, pg(s). 82
Ancient and grotesque giants banished from the world above since time immemorial, the gug is as savage a foe as it is alien. Having been exiled beneath the earth for countless generations, the appearance and morphology of the gug marks them as clearly alien. Whatever they are, they clearly have no place on Golarion. This origin would be easier to overlook if it wasn't for the occasional sign of a far greater intelligence influencing the bestial gugs.
Gugs at first glance resemble mangy, furred giants. However, even the briefest of glances reveals them to be far weirder than any sort of giant. Their most obvious difference is that they have four arms. Rather than four separate arms (which would be strange enough) gugs have their extra pair of arms growing out of what should be their elbow. Another obvious sign of their unnatural heritage is their head. Their eyes are pink, bloodshot, and are sheltered by unnatural looking bony protrusions. The most obvious deformity of the head is most easily visible when they open their mouths, which instead of opening horizontally, opens vertically and are positioned running horrifyingly across the top of the head. This turns the gug's head into a single over-sized abyss of teeth. 
Ecology & HabitatEdit
Whatever the origins of the gug and however they came to be entombed so deep below the earth, they have certainly taken to their new environment. Their horrible, flexible body structure allows them to crawl through spaces far smaller than they should be able to get through. Gugs are both carnivorous and ravenous—a dangerous combination. Though they favor the flesh of sentient creatures, gugs can survive on the subterranean flora of their homes. They are one of the few creatures that can eat undead flesh although they have an inerrant fear of ghouls.
Gugs live in the Darklands below Golarion's surface, and speak a gurgling language known simply as Gug. They normally dwell in small nomadic groups that wander the Darklands, hunting to feed their ravenous appetite. Occasionally gugs are found inhabiting huge underground cities that seem completely at odds with their bestial nature. There is no definite explanation for this, and whoever these cities were built by, they belong to the gugs now. 
- Tunnel Mobility
- Gugs have an unnatural ability to squeeze themselves through smaller gaps and tighter passages than they should be able to because of their alien body and skeletal structure.
- ↑ Sean K Reynolds, & Greg A. Vaughan. (2008). Bestiary. Skeletons of Scarwall, p. 82. Paizo Publishing, LLC. ISBN 978-1-60125-099-5
- ↑ James Jacobs & Greg A. Vaughan. (2008). Into the Darklands, p. 3. Paizo Publishing, LLC. ISBN 978-1-60125-140-4
- ↑ Sean K Reynolds, & Greg A. Vaughan. (2008). Bestiary. Skeletons of Scarwall, p. 83. Paizo Publishing, LLC. ISBN 978-1-60125-099-5