There was an asura called Gajamugasuran. He did deep penance and worshipped Lord Siva, who granted him several boons, by which he became extremely powerful.
He put the Devas and rishis to great trouble. He forced the Devas to do 1008 Thoeppukaranams in the morning, 1008 in the afternoon and 1008 in the evening. (Thoepukaranam means to sit down and stand upholding one’s ears with crossed hands, left ear with right hand & right ear with left hand).
The Devas prayed to Lord Siva to rescue them from this evil asura. Lord Siva sent Lord Vinayaga to overcome Gajamugasuran.
Vinayagar went with several weapons such as bow & arrow, sword, axe, etc. However, since Gajamugasuran had been granted the boon that he be destroyed by no weapon, none of these weapons used by Lord Vinayaga were effective.
Vinayagar broke off his right tusk and used it to kill Gajamugasuran. Gajamugasuran still rushed at Vinayaga in the form of a mooshikam (small mouse). Vinayagar crushed the ego and vanity of Gajamugasuran and sat on the mooshikam.
The humbled Gajamugasuran bowed before Vinayagar, who accepted the mooshikam as his vahana. Lord Vinayaka is the source of everlasting wisdom. He crushed ego and vanity and reduced it to the size of a mooshikam and used it as his vahana. Lord Vinayagar can be pleased by doing Thoepukaranam.