उंदीर आला ! (a mouse/rat has come!) ~ hearing this, it is not surprising that everybody runs around, trying to get away from this pest. This very pest, though, is the mount of Lord Ganesha and you will always find a mouse at the foot of a Ganapati idol, quietly sitting there. Many temples of Lord Ganesha also have a sculpture of a mouse in front of the temple gates, signifying the mouse as a guardian (one example: the Ganapati Moreshwar temple).
Do you know why the mouse, considered to be something really horrid, is Ganapati's vehicle ?
The mythological legend why the mouse is Gandapati's mount
One day during Lord Indra’s durbar, a gandharva named Krounch accidentally kicked Rishi Vamdev. The rishi was extremely angry and gave a curse, "You will become a mouse".
In the form of a mouse, the gandharva fell into the ashram of Rish Parashar. The mouse created havoc in the ashram, eating up all the food, upsetting all the vessels and other things kept in the ashram.
Seeing all this, Gajanan, who was staying in the ashram, laid a trap and caught the mouse. The mouse started begging for mercy with Gajanan. The kind-hearted Gajanan took pity on him and said “Ask for anything you wish”.
The mouse was extremely vain & scoffed at Gajanan, saying “What wish can you fulfill of mine? You ask me what you want & I will fulfil your wish!”
Immediately the clever Gajanan said, “Ok, you become my vehicle!” and in one bound leaped on to the back of the mouse.
Since then, the vain mouse became Lord Ganesha’s vehicle.