Goblin rats' shapechanging ability allowed them to assume two forms: that of a small humanoid and that of a giant rat. In rat form, they resembled a regular rat, with a length of up to 3 ft (0.91 m). Their rat form had a mouth that was packed with incredibly sharp fangs. When in humanoid form, they appeared as a diminutive and skinny "ratman", whose facial features vaguely resembled those of a rat.
While goblin rats were natural cowards and avoided combat whenever they could, a cornered or hungry goblin rat was able to unleash a deadly and vicious assault. In animal form, goblin rats used their razor-sharp teeth and claws as weapons. While in humanoid form, they could use armor and weapons, however, if they shape-changed into a rat, their armor and weapons dropped around them.
Goblin rats were patriarchal and lived in packs of up to thirty individuals. The pack was led by the strongest male. His authority could be challenged in fatal duels, the survivor becoming the new leader. During the night, goblin rat packs raided human settlements for food and treasure. Other creatures avoided goblin rats and they were considered to be symbols of cowardice in Kara-Tur. Goblin rats were utterly terrified of all species of cats.
Goblin rats lived within ruins on the outskirts of human towns and villages. They hoarded treasure within their lairs, depositing it into deep holes within.
Goblin rats were omnivores, eating whatever was available. They did not forage nor hunt, instead they raided the crops and livestock of nearby human farmers. Goblin rats, especially their animal form, carried communicable diseases such as cholera and plague. Goblin rats were shapechangers and cousins of hengeyokai.
Some locals in Kara-Tur believed that goblin rat whiskers cured baldness and made a tonic by boiling them in water. Oni occasionally hired packs of goblin rats to act as servants. No animal hunted goblin rats because their flesh was inedible.