The word Naga comes from the Sanskrit (नाग) , and nag is still the word for snake, especially the cobra, in most of the languages of India. Female Nagas are called Nagis or Naginis. In the East Indian pantheon it is connected with the Serpent Spirit and the Dragon Spirit. When we come upon the word in Buddhist writings, it is not always clear whether the term refers to a cobra, an elephant (perhaps this usage relates to its snake-like trunk, or the pachyderm's association with forest-dwelling peoples of north-eastern India called Nagas,) or even a mysterious person of nobility. It is a term used for unseen beings associated with water and fluid energy, and also with persons having powerful animal-like qualities or conversely, an impressive animal with human qualities.
According to legend, Nagas are children of Kadru, the granddaughter of the god Brahma, and her husband, Rishi or sage, Kasyapa, the son of Marichi. Kashyapa is said to have had by his twelve wives, other diverse progeny including reptiles, birds, and all sorts of living beings. They are denizens of the netherworld city called Bhogavati. It is believed that ant-hills mark its entrance.
Nagas lived on earth at first, but their numbers became so great that Brahma sent them to live under the sea. They reside in magnificent jeweled palaces and rule as kings at the bottom of rivers and lakes and in the underground realm called Patala.
It is believed that the legends of Nagas may have originated with some kind of tribal people in the past.
Like humans, Nagas show wisdom and concern for others but also cowardice and injustice. Nagas are immortal and potentially dangerous when they have been mistreated. They are susceptible to mankind's disrespectful actions in relation to the environment. The expression of the Nagas' discontent and agitation can be felt as skin diseases, various calamities and so forth.
Additionally, Nagas can bestow various types of wealth, assure fertility of crops and the environment as well as decline these blessings. Nagas also serve as protectors and guardians of treasure—both material riches and spiritual wealth.
Stories are given - e.g., in the Bhūridatta Jātaka - of Nāgas, both male and female, mating with humans; but the offspring of such unions are watery and delicate (J.vi.160). The Nāgas are easily angered and passionate, their breath is poisonous, and their glance can be deadly (J.vi.160, 164). They are carnivorous (J.iii.361), their diet consisting chiefly of frogs (J.vi.169), and they sleep, when in the world of men, on ant hills (ibid., 170). The enmity between the Nāgas and the Garulas is proverbial (D.ii.258). At first the Garulas did not know how to seize the Nāgas, because the latter swallowed large stones so as to be of great weight, but they learnt how in the Pandara Jātaka. The Nāgas dance when music is played, but it is said (J.vi.191) that they never dance if any Garula is near (through fear) or in the presence of human dancers (through shame).