A "unicorn" dog with an extra tail on its face has gone viral after being rescued from the streets, Sky News reports.

Narwhal the puppy was found with another older dog in the "freezing cold" by Missouri-based non-profit Mac's Mission.

Rescuers quickly noticed the extra tail on his face and sent him in for tests.

They found the tail was not connected to anything else and could not wag, but the organisation says the extra part makes Narwhal the "coolest puppy ever".