What happened to 'unicorn puppy' and does tail on his forehead wag?

In 2019, the world's collective heart was warmed by a rescue pup who happened to have a tail growing out of its forehead.

Narwhal, the so-called "unicorn puppy" was a biological marvel that drew adoption offers from all over the world.

But not all the messages sent to the rescue shelter in Missouri were kind.

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Because of "so many death threats and hints of danger", the founder of the shelter Rochelle Steffen decided to keep him.

Narwhal is now a happy, curious and friendly four-year-old dog.

The puppy was named after the whales known best for their tusks sticking out the middle of their heads.

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"He also absolutely loves to be outside," Steffen told nine.com.au.

"I think that is because he is part beagle. He loves to inspect the yard and run around and lay around in the grass or if there is sunshine."

Despite initial reports Narwhal was a golden retriever, his small stature indicates he is a different breed, or a mix.

But the initial publicity given to the puppy hasn't made him shy.

"He is really pretty much the sweetest guy ever and loves to go to events and work the crowd," Steffen said.

"He has four siblings in the household that he loves."

Narwhal's tail doesn't wag, and does not appear to have grown since his birth.

The cause of the forehead tail hasn't been determined, but a popular theory among biologists is that it is the remnant of a parasitic twin.

Such twins are the result of a late-splitting embryo, which results in a fully-formed animal with some leftover parts from the other.

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