Discovering you have an unborn twin living inside of you sounds like the stuff of horror movies and nightmares. In fact, having a “parasitic twin” is the plot of Stephen King’s novel "The Dark Half" and an episode of "The X-Files." But for one teenage boy in Malaysia, this isn’t a part of a fictional plot, this his reality.

Mohd Zul Shahril Saidin, 15, was hospitalized for the last four months after he complained of stomach pain. It turned out the teen had been carrying his unborn twin inside him for 15 years, and he recently underwent surgery to remove the mass in what may be the first case of its kind in Malaysia, Fox News reported.

“The fetus removed from my son's stomach was formed with organs like those of a baby -- only the nose and mouth were not complete,” said Hasmah Ahmad, the teen’s mother, according to The Mirror. This included an almost complete genitalia and hair.

This condition, known as "fetus in fetu," is rare, occurring about once in every 500,000 births. It was first described by German anatomist Johann Friedrich Meckel in the late 18th century and is characterized as having the presence of one of the twins in the body of the other. It's usually located in the retroperitoneal area — or the space between the membrane lining the cavity of the abdomen and the posterior abdominal wall.

Some scientists have speculated this condition is a result of a teratoma,which is a tumor that may contain several types of body tissue, including muscle, teeth, and bone. However, Meckel has determined that the fetus in fetu condition is the result of an underdeveloped twin. The difference between a fetus in fetu and teratoma is that a fetus in fetu displays “evidence of body plan organization, such as having vertebrae, limb buds, or organ tissue,” Medical Daily previously reported.

Saidin, who is the fifth of eight children, had an “oddly rounded shape tummy” since he was a year old, Malaysian Digest reported. However, it didn’t adversely affect him physically until he reached secondary school. His movements became more restricted due to his growing, protruding stomach. He was taken to the doctor once he started complaining of intense stomach pains. The teen and his family held a funeral for the fetus, and buried the remains in a cemetery.

The last recorded case of fetus in fetu occurred in 2015, when a 45-year-old woman discovered that a 10-centimeter mass growing on her left ovary was her unborn twin. Similar to Saidin’s case, the mass also looked like an underdeveloped human with a partially formed face, an eye, a tooth, and long black hair.