The Narwhal and Beluga Whale are (somewhat) closely related species, similar in size, shape, and color (but not tusks.)
Narwhals live near the polar sea ice. Their tusks probably help them keep breathing holes open through the long arctic winters.
The narwhal and beluga should probably be actually called "dolphins," rather than whales. They are in the superfamily Delphinoidea, which contains dolphins, porpoises, belugas and narwhals.
Of course, dolphins (and all Delphinoideas) are themselves members of the "toothed whale" sub-order of whales, but one does not normally call a bottlenose dolphin a whale!
Here is a beluga:
I think it looks much more "dolphin" than whale. :)
