Nag, and his wife Nagaina, are snakes. Specifically they are cobra snakes. Big, black cobras to be really specific.
While wandering around the garden one day, Rikki-tikki comes upon Darzee and his wife. The two Tailorbirds are in their nest crying. Being a curious fellow, Rikki-tikki asks them what is wrong.
"We are very miserable," said Darzee. "One of our babies fell out of the nest yesterday and Nag ate him."
Rikki-tikki immediately expresses his sympathies, and then asks about Nag. Rikki-tikki says that he is new to the garden and doesn't know who Nag is. Darzee and his wife are kept from answering the question, because Nag immediately sneaks up and surprises Rikki-tikki.
Darzee and his wife only cowered down in the nest without answering, for from the thick grass at the foot of the bush there came a low hiss--a horrid cold sound that made Rikki-tikki jump back two clear feet. Then inch by inch out of the grass rose up the head and spread hood of Nag, the big black cobra, and he was five feet long from tongue to tail.
Nag doesn't even let Rikki-tikki ask for his name, because the big, black cobra immediately announces who he is and how great he is. Obviously humility is not one of his characteristics.
"Who is Nag?" said he. "I am Nag. The great God Brahm put his mark upon all our people, when the first cobra spread his hood to keep the sun off Brahm as he slept. Look, and be afraid!"
Rikki-tikki is momentarily frightened by Nag, but his fear doesn't last long. Rikki-tikki soon remembers that his mother fed him dead cobras when he was baby. Rikki-tikki figures that he has nothing to fear from something that is normally his food. This entire introductory scene sets up the conflict between the likable, heroic mongoose and the detestable, evil cobras.