Fishermen catch all sorts of things from the ocean: fish, garbage, sometimes even sharks, but some Dutch fishermen weren't prepared for what they caught in the North Sea last month -- a two-headed porpoise.

The rare creature turned out to be a set of conjoined newborn twins that shared a single body.

Sadly, the small mammal was already dead when fishermen hauled it out of the chilly waters off the coast of the Netherlands. Fearing it would be illegal to keep the dead porpoise, the fishermen decided to throw it back to sea.

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But before they did, a fisherman snapped some photos of the surreal sight.

The pictures of the odd-looking creature eventually made their way to Dr. Erwin Kompanje of the Natural History Museum Rotterdam. Kompanje has been studying dolphins, whales and porpoises for more than 20 years, but he's never seen anything quite like this.

The two-headed creature "is extremely rare," Kompanje told CBS News via email. "Normal twins are extremely rare in cetaceans, conjoined twins even more rare."

This was the first case of conjoined twins in the harbor porpoise species on record. Henk Tanis/Natural History Museum Rotterdam

So rare, in fact, Kompanje said out of the 700,000 harbor porpoises in the world -- with about half of them living in the North Sea -- this was the first case of conjoined twins in the species on record.

There have been only nine other cases of conjoined twinning in a cetacean species, which consists of whales, porpoises and dolphins. The most recent, in 2014, was a two-headed conjoined set of dolphins that washed ashore in western Turkey. Like the two-headed porpoise, the dolphins were also already dead upon discovery.

"Dolphins have to be able to swim, directly after birth," Kompanje explained. "Conjoined twins are unable to swim with these complex anatomy."

In one of the photos of the two-headed porpoise, researchers were able to spot an umbilical opening, which suggests the twins died shortly after birth, according to the Online Journal of the Natural History Museum Rotterdam.

Some researchers are lamenting the two-headed porpoise was thrown back into the sea, because it could have provided some unique information about the creatures' anatomy.

"As cetaceans are mammals anatomically adapted to complete life in the sea, we are curious how the anatomy in conjoined twins is," Kompanje said. "Especially the development of the spinal musculature. Cetaceans have highly developed spinal musculature as they use their spinal column for movement."