Nag is a black cobra and is one of the main antagonists from Rudyard Kipling's short story, Rikki-Tikki-Tavi and the animated short based off it.
Nag was the husband of Nagaina and the enemy of Rikki-Tikki-Tavi the mongoose. Nag and Nagaina lived in a garden alone until a British family moved in it and built a bungalow within it. He and Nagaina were fought by Rikki-Tikki, the family's pet mongoose.
They hated him and tried to get rid of him several times. Nag infiltrated the family's house one night, so he could kill them all when they least expected it. Rikki-Tikki heard them and attacked him while he slept in the master bathroom.
While Rikki-Tikki chomped onto the base of Nag's head, Teddy's father came with his gun and shot Nag, but the cobra had been killed by the mongoose shortly before he was blown up. Nagaina, heartbroken, vowed revenge, although she eventually met her death from Rikki-Tikki as well.