Regular 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, Dr. 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.

Dr. 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.

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,” said Dr. Kilroy, who specializes in head anatomy and development at the University College Dublin School of Veterinary Medicine.

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

Dr. Casal, though, said 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.