Riti here is the story đ
There was an asura (demon) called Gajamugassuran. He did deep penance and worshipped Lord Shiva, Who, granted him several boon, by which he became extremely powerful. He was a very evil man and harmed the devas (Gods) in every possible manner. He put the devas and rishis to great trouble. He made them do all kind of immoral things. Once, he forced the devas to do 1008 Thoeppukaranams (Sit-ups) in the morning, 1008 in the afternoon and 1008 in the evening. The devas were helpless and knew no one who could help them, except Lord Shiva. The devas prayed to Lord Shiva to rescue them from the evil asuras. Lord Shiva sent Lord Vinayak to overcome Gajamugasuran and said, Please Vinayak, my son, go and defeat the evil asura that has disturbed the innocent devas.
Only you can fight him and save the devas from their plight. Vinayak agreed to help the devas and went with several weapons, such as bow and arrow, and sword, axe etc. However, since Gajamugasuran had been granted the boon that he would be destroyed by no weapon, none of the weapons used by Lord Vinayaka were effective. Vinayak broke off his right tusk and used it to kill Gajamugassuran. Gajamugasuran still rushed at Vinayak in the form of mushakam (small mouse). Vinayak crushed the ego and vanity of Gajamugasuran and sat on the mushakam. The humbled Gajamugasuran bowed before Vinayak, who accepted the mushakam as his vahana. Lord Vinayaka is the source of everlasting wisdom. He crushed ego and vanity and reduces it to the size of a mushakam and used it as his vahana (vehicle).