A narwhal resembles a very large, square-headed dolphin with mottled brown or gray skin and a large, tightly spiraling tusk growing from its head. Although the species is most readily identified by its distinct tusk, not all narwhals have them -- they are generally absent in females.
Narwhals are close relatives of beluga whales. Like their cousins, they have high, bulging foreheads and lack pectoral fins altogether. They are nowhere near as large as blue whales or sperm whales, but they are still considered whales rather than dolphins. Males grow to an adult size of about 15 feet, while females typically grow to about 13 feet.
The "horn" of a narwhal is actually a specialized tooth -- one of only two teeth in the animal's mouth. This tusk can grow to almost 9 feet in length.