Once, Arjuna was performing a tremendous penance to please Shiva. Duryodhan had sent a demon named Mooka to kill Arjuna. Mooka had disguised himself as a boar. Arjuna was engrossed in his meditation, when suddenly his concentration got disturbed by a loud noise.
He opened his eyes and saw Mooka being chased by a Kirat. In fact it was none other than ShivaShiva
Shiva is one of the Supreme Gods of Hindu Trinity that ... who had appeared in the guise of Kirat. Both Arjuna and Shiva struck the boar with their respective arrows at the same time.
A disagreement broke between Arjuna and Kirat (Shiva’s avatar) over who killed the boar. Arjuna challenged Shiva for a dual. Shiva was very much impressed by his valour. He revealed his real identity. Arjuna was very ashamed that he fought with Shiva but Shiva consoled him and gave his weapon Pashupat to Arjuna.