A rat
A rodent is a small animal belonging to the mouse or rat family and is usually considered a pest.
On Dimorus, the indigenous rodent creatures use deadly poisoned darts as weapons. (TOS: "Where No Man Has Gone Before")
Julian Bashir compared the situation in the Wadi labyrinth to a laboratory rodent in a maze. (DS9: "Move Along Home")
In 2370, Lieutenant Commander Data remarked that his pet cat, Spot, had never seen any form of rodentia after he witnessed her twitching and breathing rapidly, which he concluded suggested anxiety. (TNG: "Phantasms")
In 2371, after she was captured by the Vidiians, Lieutenant B'Elanna Torres explained that Klingons found honor 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")
In 2373, Q called Neelix a "bar rodent" as he ordered another drink. (VOY: "The Q and the Grey")
Similarly, in 2378, Q Junior called Neelix a "kitchen rat". (VOY: "Q2")