1
A whale is a marine mammal of the order Cetacea.
2
The order Cetacea consists of three families, baleen whales, sperm whales, dolphins, and it's in this last group that narwhales are placed.
3
From this I finally concluded that it belonged to the branch Vertebrata, class Mammalia, subclass Monodelphia, group Pisciforma, order Cetacea, family .