We know that Ganesha's vehicle is a mouse. The sight of the huge potbellied God on a tiny mouse is an incongruous sight, and many of us would have wondered how the tiny mouse bears the weight of the God! Let me tell you how Ganesha got such a vehicle.
The one who has the mouse as his vehicle
(image from the internet)
There was once a Gandharva (a celestial being) named Kroncha. Now, Gandharvas are believed to be extremely good looking and talented. This frequently makes them quite vain and proud. This was the case with Kroncha too. In the court of Indra, he once met the sage Vamadeva, and filled with pride, insulted him. This incited the anger of the sage, who cursed him that he would lose the good looks he was so proud of, and become a mouse!
Kroncha found himself turned into a mouse, and ashamed of his appearance, ran away. He entered the ashram of Sage Parashara, and, true to his nature in his new form, started creating havoc. Meanwhile, Ganesha was paying a visit to the sage. The sage’s disciples approached Ganesha and asked him to put an end to the mouse which was wreaking havoc in their well kept ashram.
Ganesha at once realized the true identity of the mouse. Using a rope, he lassoed the mouse, and mounted it. Contact with the divine body of Ganesha made Kroncha at last realize the error of his ways. Besides, Ganesha was so heavy that he felt himself being crushed. He prayed to Ganesha, “Lord, please forgive me for my sins. I have learnt my lesson. Please accept me as your mount and release me from this misery!” Ganesha relented and made himself light enough for the mouse to bear his weight. And thus did he become Mooshika Vahana – the one with the Mooshika (mouse or rat) as his mount or vehicle.