A rat
A rat is a small rodent that is larger than a mouse. It can be found on a variety of worlds.
In 2369, Odo disguised himself as a rat in order to overhear a conversation betwenn Lursa, B'Etor and Tahna Los. (DS9: "Past Prologue")
A year later, Odo snuck aboard a Kressari freighter and disguised himself as a rat again to investigate allegations by one of Quark's contacts that the Kressari were arming the Bajoran extremist group known as "The Circle". (DS9: "The Circle")