Dutch fisherman inadvertently caught a two-headed harbour porpoise as bycatch in a beamtrawl net in the North Sea last month. This is believed to be the first known case of a two-headed porpoise, according to National Geographic.

The fishermen threw the porpoise back into the sea, thinking that it would be illegal to keep the porpoise. But they did take these photos, allowing researchers at least a glimpse of this bizarre case. Scientists could determine that the mammal was newborn and a male from the photos.

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A study published in the Journal of the Natural History Museum Rotterdam calls this an extremely scarce event, stating that there are only 10 known cases of conjoined twinning in any cetacean species. Even normal twinning is rare among cetaceans, with only a rate of 0.5 percent in small cetaceans, according to the same report.

The harbour porpoise — Phocoena phocoena — is one of the smallest marine mammals at about 4 to 6 feet, and is most commonly found in the north Atlantic and Pacific oceans. There are about 700,000 harbour porpoises worldwide, with about half living around the North Sea.