Mahabharata
In the great epic Mahabharata, the depiction of nagas tends toward the positive. An epic calls them "persecutors of all creatures", and tells us "the snakes were of virulent poison, great prowess and excess of strength, and ever bent on biting other creatures" (Book I: Adi Parva, Section 20). At some points within the story, nagas are important players in many of the events narrated in the epic, frequently no more evil nor deceitful than the other protagonists, and sometimes on the side of good.
The epic frequently characterizes nagas as having a mixture of human and serpent-like traits. Sometimes it characterizes them as having human traits at one time, and as having serpent-like traits at another. For example, the story of how the naga prince Sesha came to hold the world on his head begins with a scene in which he appears as a dedicated human ascetic, "with knotted hair, clad in rags, and his flesh, skin, and sinews dried up owing to the hard penances he was practising." Brahma is pleased with Shesha, and entrusts him with the duty of carrying the world. At that point in the story, Shesha begins to exhibit the attributes of a serpent. He enters into a hole in the Earth and slithers all the way to bottom, where he then loads the Earth onto his head. (Book I: Adi Parva, Section 36.)
Read more about this topic: Phaya Naga