Partially paraphrased from the Dwarf Fortress Wiki
Gorlaks are small humanoids encountered in cavern areas. Despite their appearance, they are mostly harmless, although they generally prove to be an annoyance due to civilians canceling jobs, and occasional weak attacks against animals or sleeping dwarves. They are easily dispatched by even a novice military dwarf and/or most civilians, as they have soft bodies and thin skulls.
While gorlaks are, for the most part, solitary creatures, they have a semi-random "mating season", in which all the gorlaks in a small region congregate in a single area. The exact timeframe of this mating season varies depending on the region. However, the place of nesting remains the same every year, no matter the season. It is unknown exactly how the gorlaks themselves know when it is time to mate, but it is speculated by biologists that the gorlaks, with their large noses, are able to pick up pheromones that the dwarven nose cannot.
The nest itself is usually circular in size, preferably with a stream or shallow pool of water in the middle, so that pregnant gorlaks don't have to travel far to drink. This area is usually well-guarded, and at the first sign of intruders the gorlaks are capable of disappearing into the shadows. Passersby, unless well-trained, would likely never realize the gorlaks were there, or had ever been there, at all.
After the mating season, the gorlaks generally remain in the nest for another year while the young gorlaks mature. They grow quickly, reaching a fifth of their adult size by the end of this year. It has been suggested by various zoologists that they do this in order to protect the young gorlaks from outside danger, seeing as the nesting grounds are generally a very safe place.
While gorlaks are common in the region itself, Spearbreakers does not have a naturally-occurring gorlak population beneath it. Mr Frog, however, took the liberty of setting up an experiment in the Gorlak Caves to see if they would respond well to selective breeding and growth acceleration. This was done in an attempt to turn them into war animals. The experiment was held far closer to the surface than gorlaks typically nest, as it was connected to Mr Frog's Mossy Tunnels, and the creatures took advantage of a shaft of light to make their nest safer with increased visibility. This led to the Moonlit Grotto becoming the center of their nest.
Hans and Urist Okablokum made the mistake of unknowingly hiding their weapon cache of subrailguns in the Gorlak Caves, after Hans tunneled into it following the Infiltration Mission, and they lived there for some time. Mr Frog had since abandoned the experiment, but through use of his cameras was able to take note that Hans and Urist had taken up residence in the area. When Vanya later found herself at The Forges, Urist took her there to keep her safe, but they wandered through the Moonlit Grotto. This led to the gorlaks becoming hostile, and they would have killed Vanya had Mr Frog not intervened with his serrated disc launcher.