After the demon (Asura) Mahisha (Mahishasura) was destroyed by Durga, his son Gaja (Gajasura) sought revenge. He meditated on Brahma who granted him a boon. Gaja asked for invincibility and soon tyrannized the worlds.
One day he went to Kashi, the residents fled. The Gods asked Shiva to take action. A huge fight ensued, in which Shiva stabbed the demon with his trident. At that moment Gaja recognized the glory of Shiva, praised him and Shiva, out of sheer emotion, granted him a favor.
Gaja cheered: Please wear my skin. It was consecrated by your trident (trishula). It fits you like a glove and is comfortable to wear.
Shiva pulled Gaja’s skin on while the demon died. Peace returned to the worlds.
See also Shivas Insignia.