Dundubha

A snake, the cursed sage Sahasrapat, who once frightened his friend Khagama with the dummy of a snake and was cursed by him to become a snake until he would meet Ruru, who would free him from the curse.

To read the story about Ruru visit
http://sushmajee.com/mahaabhaarat/mbh-1-ganguli/g-0-prolog/5-chyavan-ruru.htm