"Rikki-Tikki-Tavi" is available to read for free on the website linked below. Here is a brief summary.
A family newly moved into a home in India find the title character, a mongoose, almost dead from a "summer flood." They take him to their house and dry him out, and he becomes a pet for the child, Teddy. The father realizes that the mongoose will be useful in protecting Teddy from snakes, and later Rikki-tikki kills a smaller venomous snake. Two cobras, which had been living in the house and garden while it was empty, plot to kill the family and hatch their own children there. Rikki-tikki defends the family from Nag, the male cobra, who is killed by the father's shotgun; Nagina, the female cobra, vows revenge. Rikki-tikki tricks Nagina into leaving her eggs, and breaks all of them except one; when Nagina threatens the family, Rikki-tikki uses the last whole egg to draw her away and kills her underground after a long battle. Rikki-tikki remains with the family, patrolling the garden "till never a cobra dared show its head inside the walls."