Mouse

There was a celestial musician-god by the name Krauncha. One day, in the court of Lord Indra, Krauncha accidentally stepped on the foot of Muni Vamadeva, who (as all Munis), got enraged and cursed Krauncha to become a mouse. However, Krauncha became a huge mountain-sized mouse and ended up damaging everything in its path. Once, he ended up stepping on the ashram of Maharshi Parashar, with whom Lord Ganesha was staying, and destroying it. Lord Ganesha, inorder to teach Krauncha a lesson, unleashed his pasha (noose) on Krauncha which ended up looping around the mouse and bringing him to Lord Ganesha's feet. Ganesha then said something like, "Krauncha...you have caused a lot of trouble and you deserve a severe punishment. But since you ask for my forgiveness, I will pardon you and use you as my vehicle". However, when Ganesha mounted on Krauncha, he couldnt bear the weight of Lord Ganesha. Krauncha pleaded for Ganesha to become light-weight so that he could support him. Lord Ganesha obliged and since then, has been using the mouse as his vehicle

http://www.biodiversityofindia.org/index.php?title=Why_Lord_Ganesha_has_a_mouse_as_his_vehicle