Chances are you've seen this viral image of a cougar and its "parasitic twin" doing the rounds recently. The photo of the big cat with a strange toothy deformity protruding from its head was posted by a hunter in Idaho, who wishes to remain anonymous, early last week. In the days since, it's caused quite the stir. The most likely explanation, however, is that the bizarre growth is actually made up of the mountain lion's own flesh, not that of a long-lost sibling. 

Image: Backcountry Brotherhood, used with permission from the hunter.

Upsetting as this may be to some, it's important to note that the cat was hunted legally under the regulations set by Idaho Fish and Game (IFG). "Like elk and mule deer, cougars are considered big game in the state of Idaho," explains IFG Southeast representative Jennifer Jackson. "We have an annual hunting season for them [which runs August 30th to March 31st], but there are a lot of rules that have to be be followed."

All catches must be submitted for a mandatory check by wildlife officials, who conduct a size evaluation, remove a tooth from the skull for analysis and mark the hide of the animal for state records. "Here in the region, we also have a female quota," adds wildlife manager Martha Wackenhut. "Once 22 females are taken, we end the season completely and females with kittens are strictly prohibited."

Some have suggested the growth is a parasitic twin, which occurs when a developing embryo does not separate from its in-utero sibling. In the most dramatic incarnation, known as "fetus in fetu" or "vanishing twin", the developmentally abnormal parasitic twin is completely encapsulated within the torso of the otherwise normally developed host twin. We've seen the crew of both Grey's Anatomy and House take on a fetus-in-fetu case, but in reality the condition is very, very rare – even more so in non-human animals. 

Image: Backcountry Brotherhood, used with permission

The more likely explanation for the cougar's Hellboy-esque horns? A teratoma a congenital defect that occurs when the embryo's own germ cells (reproductive cells that can form various kinds of tissue) end up in the wrong place. The resulting tumour can contain anything from hair, to bone and muscle. The mountain lion's growth even sprouted gum tissue and whiskers. 

This condition is far from common, but, over the years, it has been documented in various mammals, including horses, dogs, monkeys and other cats.

U.S. Fish and Wildlife hopes the hunter will bring the big cat in for scans, so that it can be studied further. If that doesn't happen, we may never know with total certainty what caused the mutation.  

"To me, the most amazing thing is that the animal was getting along well enough and was healthy enough to be harvested in the wild like that," says National Park Service cougar biologist Seth Riley. "We have certainly never seen anything like it!"

Image: Idaho Fish and Game