A Rafalian mouse was a small animal.
In 2372, Odo took the form of a Rafalian mouse as he practiced his shape-shifting, following a complaint he received from Quark, whose quarters were directly below Odo, regarding all the noise he was making as a Takaran wildebeest.
Quark later complained that even as a mouse, he could still hear Odo's tiny little feet skittering back and forth across the floor. He added that sometimes he could even hear Odo oozing around up there as well. (DS9: "Crossfire")