This case of a single body and two fully grown heads, also known as partial twinning or parapagus dicephalus, is extremely rare -- only 9 other cases of conjoined twins in cetaceans, the group of animals which includes whales and dolphins, are known. The first known case of a two-headed harbour porpoise was documented last month when Dutch fishermen in the North Sea accidentally caught the anomalous creature as bycatch in a beamtrawl net. Believing it would be illegal to keep the mammal, the Dutch fishermen threw it back into the sea, but luckily, they took photographs that specialists could examine so they could shed light on the mysterious creature. The dorsal fins were not yet erect, the rostrums (or top of its heads) still contained hairs, and it still contained an umbilical opening, indicating the porpoise died shortly after birth. “Even normal twinning is rare; there’s no room in the womb of the mother for harboring more than one baby.”

And the few specimens that had been found were usually undeveloped fetuses, like the “double monster” – one head and two bodies – found in the womb of a dolphin caught by herders off the coast of Japan in 1970.
The deceased body of a rare two-headed porpoise is located for the first time in the North Sea by Dutch fishermen.