According to Hindu mythology there are two planets called Raahu and Ketu in Indian astrology. These two have no counterparts in western astrology. These two are neither gods, nor demons, nor planets. 

At the time of churning Ksheer Saagar (Milky Ocean) one Raakshas had a share of Amrit, the Divine Nectar, cleverly hiding in disguise of a Devtaa, but the Sun and the Moon noticed him doing that and reported that to Mohinee (Bhagvaan Vishnu) who was distributing Amrit (nectar or Ambrosia) to Devtaa and liquor to Raakshas. 

Vishnu immediately slit that Raakshas's throat with his disc. Since the Amrit had already gone down the throat of Raahu, he did not die, but he became immortal. But by slitting throat Rahu became a two-piece entity, one the head, named as Raahu and the other one his trunk (body) named as Ketu (tail) and both parts of his body were also immortal. 

Because the Sun and the Moon reported Raahu's wrongdoing, he prays to subsist on eating away the Sun and the Moon. That prayer was granted by Vishnu subject to the condition, that this eating will be done only occasionally. Those are the occasions of eclipses of either Sun or Moon - solar eclipse and lunar eclipse. Recently these two have been given names in astrology as 'Dragon with Head' for Raahu and the 'Dragon with Tail' for Ketu.