A so-called 'unicorn' dog, who has an extra tail on his face, has gone viral after being rescued from the streets in the US.

Narwhal the puppy was found with another older dog by a Missouri-based non-profit called 'Mac's Mission'.

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

Image: Facebook/Mac the pitbull

They found the tail was not connected to anything else and could not wag - but the organisation said the extra part makes Narwhal the "coolest puppy ever!"

In a post on its Facebook page, the charity said: "This is a true experience of having 'extra parts' during assembly.

"Dr Heuring said there is no real reason at this time for the unicorn tail to be removed (and we all wish it wagged)!!

"Pretty sure the staff at Cape Small also gushed all over this little nugget!

"The unicorn face tail does not bother Narwhal and he never slows down just like any normal puppy.

"He seems completely healthy other than some usual puppy worms he got meds for."

The charity added that he is not yet available for adoption as they want him to grow a bit more and "make sure the tail doesn't become a bother or problem."

Image: Facebook/Mac the pitbull

"Right now he is pretty much the most unique amazing example of what we do here and we are so thankful to have the chance to be part of his journey."

In a post written in the first person, Narwhal said: "I don't understand what viral is but my foster mama said my story being viral helps all our special needs dogs here at the mission.

"You are welcome. I am super excited for being the poster child for 'special is awesome'".