Here are a couple of things we learn about Nag when he first arrives on the scene:
Given that, care to take a guess who's the story's antagonist? Yeah, it's Nag and his wife, Nagaina. Other clues to their villainy are Nag's attempt to kill the entire family in their sleep, and Nagaina's desire to kill Teddy as an act of vengeance.
And the fact that they are snakes. Honestly, when was the last time you experienced a story with snakes or snake-like characters as the protagonist? Exactly.