A 10-week-old puppy with a tail attached to its head has been rescued after it was abandoned and became a stray dog.

Staff at Mac's Mission in Missouri, who rescued the abandoned unicorn-looking pup, named him Narwhal after the type of whale that lives in Arctic waters and has a protruding tusk.

The Mission takes in dogs with all types of special needs such as deformities, clefts and trauma to prevent them from being put down.

Rochelle Steffen, who runs Mac's Mission, said Narwhal's tail would remain on his head as it causes him no pain.

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X-rays had showed his secondary tail, about a third the size of his actual tail, was not connected to anything and served no purpose other than to make him the "coolest puppy ever".

Vets have said there is
Vets have said there is "no medical need to remove" Narwhal's extra tail. Photo / Mac's Mission

Steffen said the moment she saw the "unicorn puppy" two thoughts ran through her head.

"I looked at him, and I was like, 'This is the most ridiculously adorable puppy I've ever seen,'' Steffen told Today.

"Then my million-dollar question was, 'Did the tail wag?'''

Steffen revealed the pup's tail doesn't wag, and said she initially wanted to call him wipers for a laugh.

The dog has become an instant sensation on the organisation's Facebook page.

An adorable
An adorable "unicorn" puppy named Narwhal has become a sensation for the tail growing off its face. Photo / Mac's Mission

Dr Brian Heuring, who examined Narwhal's tail said it's the first time he's ever seen this on a dog.

"I've been a veterinarian for 16 years, and I've never seen anything like this,'' Heuring told Today. "It certainly looks just like a tail, but we did X-rays, and it's all just skin."

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Mac's Mission has been inundated with requests from people looking to adopt Narwhal, however, he is not yet old enough to be released.

The dog will stay at the shelter for a least a few more weeks and possibly months in order to monitor whether any complications develop with the tail on his face.