Is there any reason for peacock as Vahana of Lord Karthik? What is the story?
In the legend given in mahAbhArata, peacock was the son of garuda and cock was the son of aruna. When kartikeya was born every god gifted him something or the other, where garuda gifted him his son ‘peacock’ as his vehicle, and aruna gave him his son ‘cock’ which became kartikeya’s banner.
The adorable chastiser of Paka, O tiger among kings, gave unto Guha (Kartikeya) a dart for the destruction of the enemies of the gods. That dart produces a loud whiz and is adorned with many large bells. Possessed of great splendour, it seemed to blaze with light. And Indra also gave him a banner effulgent as the morning sun. Shiva gave him a large army, exceedingly fierce and armed with diverse kinds of weapons, and endued with great energy begotten of ascetic penances. Invincible and possessing all the qualities of a good army, that force was known by the name of dhananjaya. It was protected by thirty 30,000 warriors each of whom was possessed of might equal to that of Rudra himself. That force knew not how to fly from battle. Vishnu gave him a triumphal garland that enhances the might of the wearer. Uma gave him two pieces of cloth of effulgence like that of the Sun. With great pleasure Ganga gave unto Kumara a celestial water-pot, begotten of amrita, and Brihaspati gave him a sacred stick. Garuda gave him his favorite son, a peacock of beautiful feathers. Aruna gave him a cock of sharp talons. The royal Varuna gave him a snake of great energy and might. The lord Brahma gave unto that god devoted to Brahman a black deer-skin. And the Creator of all the worlds also gave him victory in all battles.