A puppy with a tail on his face gained viral fame with Twitter commenters pledging, “I would die for Narwhal.”

The rescue mutt was named for a marine mammal with a tusk that sticks out of its face. But instead of a tusk, Narwhal the puppy has a miniature tail flopping between his eyes.

A Missouri shelter called Mac’s Mission, which specializes 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, said Margret Casal, a professor at the University of Pennsylvania School of Veterinary Medicine. The tail is probably Narwhal’s parasitic twin.

Identical twins form when an embryo splits in half very soon after fertilization. Sometimes, this split happens too late in a pregnancy and the halves don’t fully separate, leading to conjoined twins. Even more rarely, Casal said, 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 highlighted 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 said this could suggest a twin’s rear end on Narwhal’s face.

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

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

He said a parasitic twin might explain Narwhal, but that it’s impossible to know for sure. 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 said.