A rodent is a small animal belonging to the mouse or rat family and is usually considered a pest.
In 2371, after she was captured by the Vidiians, Lieutenant B'Elanna Torres explained that Klingons found honour on the battlefield, not as guinea pigs in a laboratory. (VOY: "Faces")
Later the same year, Doctor Ma'Bor Jetrel visited Neelix aboard the USS Voyager in order to screen him for a fatal blood disease, caused by a weapon of mass destruction, created by Jetrel himself. Neelix refused saying that Jetrel "would have to find another laboratory rodent to help him in his experiments". (VOY: "Jetrel")