A puppy with a tail on his face gained viral fame this week.

“I would die for Narwhal,” a number of Twitter commenters pledged.

The rescue mutt was named after a marine mammal with a single tusk that sticks out of its face. But instead of a tusk, Narwhal the puppy has a miniature tail flopping between his eyes. Scientists don’t agree on how the unusual heart-stealer came to exist.

A shelter in Missouri called Mac’s Mission, which specialises in what it calls “janky” dogs, took in the abandoned puppy. Staff were disappointed that Narwhal’s extra tail didn’t wag. But the appendage didn’t seem to bother the otherwise normal, healthy puppy, and a veterinarian said there was no need to remove it. An X-ray showed no bones.

The likeliest explanation for how Narwhal got his face tail is not all that cute, says Margret Casal, a professor at the University of Pennsylvania School of Veterinary Medicine. The tail is probably Narwhal’s parasitic twin.

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Regular identical twins form when an embryo splits in half very soon after fertilisation. Sometimes, this split happens too late in a pregnancy and the halves don’t fully separate, leading to conjoined twins. Even more rarely, Casal says, the late split is asymmetrical, meaning one side of the embryo grows into a fully formed individual and the other becomes an extra body part.

Casal highlights a little mohawk of backward-growing fur above Narwhal’s face tail, similar to the crest on a dog such as a Rhodesian ridgeback. She says this could suggest a twin’s rear end on Narwhal’s face.

David Kilroy’s first impression of Narwhal was different.

“At first I thought that it was a bit of clever computer work and not real,” says Kilroy, who specialises in head anatomy and development at the University College Dublin School of Veterinary Medicine.

But after looking at the photos and X-ray, he says, “it looks like some weird outgrowth of skin. Although something so large and strange would be most unusual.”

Casal, though, says the bottom of a spine can’t develop bones without signals from the top. So if Narwhal’s appendage is a parasitic twin, it might make sense that it never grew bones.

Unlike in humans, identical twins are very rare in dogs, which are typically born in litters, Casal says. So a dog with a parasitic twin is “really super, super rare”.

But it’s not unheard of. In one case, a puppy had an extra pair of hind legs growing from its belly. Parasitic twins, like conjoined twins, can occur in humans, too.

Animals are sometimes born with more extreme spare parts, like an entire second head. Two-headed calves occasionally show up in headlines, though they usually die soon after birth.

Snakes, too, can hatch with two heads. In a 2007 paper, a herpetologist, Van Wallach, summarised nearly a thousand reported cases of two-headed snakes. The two heads are almost always next to each other, he found, but occasionally stacked. Many factors can lead to two-headed snakes, including cold temperatures when eggs are incubating. Most two-headed snakes die right away, but a few live to adulthood.

Wallach had a pet two-headed snake called Brady and Belichick that grew to healthy adulthood. Both heads ate normally. But the head that finished eating its mouse first would then attack and chew on the other head, as Wallach described in his 2012 paper, “Two-headed Snakes Make High Maintenance Pets”.

A calf or snake’s second head can arise from a parasitic twin. Or an extra head can form when something goes wrong during a single individual’s development. For example, certain genes act like stage directors in a developing embryo, making sure everything ends up in the right place.

“If you get a mutation in one of those genes then you can get bizarre duplications,” like two heads, Casal says. “Or, what we see every once in a while in dogs or cats is they can have, for example, two penises.”

Michael Levin, who directs the Allen Discovery Centre at Tufts University, says that while Narwhal is a cute example of development gone awry, “I’ve seen a lot weirder”.

Levin studies how signals between cells, especially electrical signals, help to organise a whole animal into the correct shape. Researchers in his lab have created worms with four heads, tadpoles with eyes on their backs and six-legged frogs.

While Levin thinks a parasitic twin might explain Narwhal, he says it’s impossible to know for sure because of the complex processes that organise bodies even in simple creatures, like flatworms. Chemicals and other factors in a developing animal’s environment can make these processes go wrong in countless ways.

“There are massive gaps in our understanding,” he says.

Scientists are still trying to answer major questions about how a blob of cells turns into a complete animal of just the right size and shape, with different kinds of parts in all the correct places.

Levin says “it’s a miracle it comes out right most of the time”.